Why are some people anti-sippy? Well, one point sometimes made is that they aren't necessary. The alternative name "trainer cup" implies that they are an essential step to learning how to drink, but this is of course nonsense; babies learned to drink from cups just fine before sippies were invented, and getting water out of a sippy involves an entirely different set of skills compared to drinking from an open cup. The real raison d'etre of sippies that they are spillproof and therefore a convenience to parents. Here in Japan, most daycare nurseries apparently insist that toddlers over 12mo use open cups only--a fact which amazes some Westerners when they first see all these tiny tots independently drinking from these little cups (without spilling... no doubt due to that mysterious ability that daycare workers have to get children to do things that they would never ever do at home).
Some lactivist-y people are anti-sippy because of fears that babies who start to enjoy the autonomy of wandering around with a cup will be more likely to self-wean early from the breast. Skipping sippies is relatively popular among baby-led weaners, which is a bit surprising, really; sippies actually allow early self-feeding and autonomy earlier and it's a rare baby who can manage an open cup independently at 6mo. However, mainstream pediatricians and dentists also tend to recommend skipping/severely limiting sippies. Extensive use of these cups has been linked with speech problems, possibly because in order to get liquid out of them--especially ones with valves--you have to push your tongue into a peculiar position which isn't used in the formation of sounds. And the very spillproof-ness of these cups can be a bit of a menace. Parents are tempted to put juice in and let toddlers wander away from the table with them (no way would you do that with an open cup, unless you like having disgusting sticky all over your house); sucking on the cup keeps kiddo quiet, so you let them have it a bit longer; and then you find yourself giving them a sippy with juice to stop them wailing in the car seat, and so on, and before you know it, the kid is spending half the day with a chewed-up plastic stub in their mouth and juice/cow's milk pooling around their teeth. The other thing about sippies is that they are often a bit... gross. They get all chewed-up, they have all these little cracks and crevices for germs to breed, they get thrown around and dropped and picked up and put in the mouth again and discovered under the car seat covered in two weeks' worth of fluff and lint and dog hair... you get the picture.
The case for sippies
That said, when at 9mo Little Seal finally took to one of the sippy cups we offered (after I got frustrated one day and ripped the valve thing out... I felt vaguely guilty afterwards, as if I'd vandalized the cup, then an online search revealed that loads of parents do this), the dreaded sippy also turned out to be useful in its own way, and I can't quite imagine never using anything but an open cup with a toddler.
Why? Well, for one thing, kids don't seem to drink quite as much from open cups as they do from sippies. I've found it useful to "add in" a little drinking from the sippy at times when it just seemed like she needed more fluids. Also, I'm not sure how you are supposed to nightwean if you never do bottles or sippies; how do you give them a sip of water if they do wake up? Open cups of water in a dark bedroom when you're tired sound like a disaster waiting to happen. Finally, I'm told that car-using parents find a sippy of water useful when they are on the road.
Cups, cups, cups
Back when we were in the bottle-refusal trenches, I did briefly wonder whether the famous Doidy cup (a funny little open cup with tilted sides) might be the answer. I think I'm glad I didn't bother, since the impression I get from Mumsnet is that there are a lot of Doidies gathering dust in people's cupboards. The Doidy's tilted sides allegedly make it easier for babies to control the flow of liquid and thus drink independently; the reality seems to be that the vast majority of babies need some help with a Doidy until toddlerhood, much like with an ordinary open cup (as one Mumsnet poster put it "The myth of the Doidy cup: 4 month old baby sips delicately at the clever slanty cup of healthy beverage. The actuality of the doidy cup: 15 month old baby tips contents of clever slanty cup all over own chest. Cries. Change clothing and repeat. Buy cup with spout.") Also, in my experience, babies spill from open cups mainly because a) they plonk cups down clumsily and b) they think it's fun to chuck water everywhere... not because they can't control the water flow. I think if you use any type of open cup with a baby, realistically you need to be prepared to help them out with it for quite a long time unless you like soggy children and drenched floors.
A lot of parents seem to be using straw cups these days due to concerns about sippies. If I could do things again, I think I would have given these a try with Little Seal. It's handy if a baby can drink from a straw early on. That said, the other issues of sippy cups--grunge/germs, temptation to let the kid sip juice or milk all day--still remain with straw cups, so I think there's still a case to be made for introducing an open cup in the first year as well. Also, I wonder how long it'll be before "they" (dentists, pediatricians, whatever) discover some sort of problem caused by sucking on a straw all day long...
My verdict
So, all in all, going straight to an open cup--for 90% of water consumption, anyway-- turned out to be quite a nice way of doing things. You do have to supervise more and have a little more tolerance for spills--but then, if you're only doing water in the cup, this isn't usually a real problem. And I love not having to drag grotty sippies around with me. Some restaurants only have thin glasses which a young child could bite through causing serious injuries (sturdy glasses are OK); in these cases I ask for a mug or a straw, or just feed water on a spoon. The other useful thing I worked out was to keep the cup out of reach, offer it at intervals and teach Little Seal to point to it when she wants to drink. If the cup is right there in front of a toddler, the temptation to dump the contents all over the floor (just to check that gravity is still working) can become irresistible.
The sippy in our house is only for water; cow's milk comes in an open cup. However, I suspect that using an open cup only for milk is probably only practical if you are doing extended nursing, with cow's milk as a sort of supplement; if I were only doing cow's milk/toddler formula, I think I'd use a sippy/straw cup--not sure a toddler would reliably drink enough milk from an open cup. I do think that the pediatrician's rule of "juice only in an open cup (with or without straw)" is a good rule to stick to, as it means that juice stays in its rightful place as something sipped occasionally at mealtimes, not an all-day pacifier. You see, as it happens I'm awful with the TV... you know, putting it on for a few minutes (cue mummy-guilt spasm) multiple times a day when I need to immobilize Little Seal or get essential stuff done, so I think I'm probably better off not having the temptation of juice or milk in sippies for keeping Little Seal quiet; I bet I wouldn't be self-disciplined enough to restrict usage.
To sum up; I think using open cups as the main drinking device for older babies and toddlers can work, but better to be a bit flexible and consider adding in a sippy or straw cup when it seems to make sense. Happy drinking!