I think that part of limiting TV should be limiting what is watched. Cartoon Network, for example, has very few shows I would consider educational. Nick Jr, on the other hand, has a LOT of good preschool level shows on it, which are educational.
My kids watch TV as long as the chores are done, and the homework is done, etc. I prefer they not sit in front of a TV all the time, because even though the mind needs exerscize, so does the body and sitting in front of a TV for 5 hours isn't good for the body.
We watch a lot of Nick Jr, Noggin, and channels such as the History Channel, National Geographic Channel, Animal Planet, etc, as well. Those are great learning channels, even for adults.
About the only thing I would say is to limit anything innappropriate. A tv can be used as a learning tool, but that shouldn't replace things like reading to your child, or taking your child to things that are outside the home- parks, musuems, the beach, etc.
Your children sound like they are doing great for their age, just don't forget that there are other things out there where a child should experiance it first hand to learn it best.