IMHO, I don't think there's much truth to it. I've heard folks say that, but I've never seen anything scientific to back it up. I think kids develop at their own pace, and the tiny part of the day that they spend sucking on a cup probably doesn't impact that. My DS still uses the non-spill sippy cups at 26 mo and always has, and his verbal skills have been above average since the start and his pronunciation is normal for a 2 yr old. Just my 2 cents.