There's a sense in which some imagination could be lost with television, sure. I'm prepared to buy that argument - or at best part of it. However, there's another side to that argument that supporters with this theory may not be seeing, and that's the concept of toys deriving from said television shows.
A great cartoon won't entertain us making us laugh, nevertheless it can actually enhance our creativity when developed the correct way. For instance, if we're talking about a cartoon when the main character gets himself into all kinds of crazy situations, the little one playing with the toy can continue to certainly use his / her imagination to pretend their toy in similar binds. Not only does that make use of their imagination, but it takes problem-solving skills to get the character OUT of the messes. Add into an antagonist or two, and we're good.
If you have good writing on the show - especially when humor can be used - there is a sense where the adventure is never finished, and that can spill over into toy play. In case there are lessons taught within the show, then just maybe our children are absorbing these lessons, are associating the characters with all the themes, and will play with their cartoon toys in a similar fashion. It's just up to us as a parent and guardians to find yourself in our children's activities and make them learn good play habits.
Cartoons can certainly be abused and made to do something as pacifiers, but we need to remember that entertainment is entertainment and possesses been a great source of imagination and creativity for as long as man has been on the planet. It's what we do with it, how we limit the intake, and the way we expand about it that is the key.