My main gripe against sippy cups is that I would actually have to go out and buy one and put it in one of my already-overpopulated cupboards. Also, I want my kids to know that if they are thirsty they should go get a drink, not carry one around with them all day. That's just my preference, though. There are other more scientific reasons to consider not using them.
- require a child having to relearn to drink from a regular cup at some point, possibly after forming a comfort attachment to the sippy cup. Doesn't sound fun to me.
- can affect speech and mouth development, and oral hygiene
- are hard to keep clean, and contain plastics (which are now known to leach toxins)
- teach breastfed babies a different way of sucking that can cause pain to the mother while nursing
Here is a good article by a pediatrician