It's wrong because those who do so, believe that one pokemon is stronger than all others. And what's worse, they are not wrong, because due to how the game is made, it is perfectly possible to beat a lvl 60 Blastoise with a level 80 Charizard.... Anyway, doing that is easy, it's easy to beat a level 10 pokemon with a level 30 pokemon, even if it's a Caterpie at 30. But it doesn't teach you how to play good, it teaches you that you don't need to know how to play good, all you need is to stuff your one pokemon with experience and voila. Playing like that will make you dumb, you'll pass the whole game believing that one pokemon can solve anything and disappointed, you will leave the game never to be picked up again. I don't claim that I am 100% right but I had a certain friend and he did that :/
One more thing. If you look at Charizard's learning set in generation 1, and compare it to Pidgeot's, you're gonna see that game developers actually anticipated using only your starter pokemon the whole game, because Charizard, in comparison to many other pokemon is better, since he has a much better learning set (among other things). That fact sucks.