There's no problem with it, really. I know some people who've roleplayed for over a decade and always play one certain archetype of character, and it's just fine. It can become problematic if you have a steady group of friends you RP with and you always just port in the same character, because they'll probably get bored of interacting with essentially the same character over and over, but in a forum roleplaying environment you could probably get away with it without many people ever knowing you just play the one thing.

I can't really do that, but it all comes down to personal preference. I used to try to do it by always playing paladin type guys, but after a while I realized that having the same kind of character over and over contributed a lot to me getting quickly burnt out on roleplaying in general. After I started to switch things up and play female characters and charismatic charmers and magic users and so forth I found my interest in roleplaying to have much greater longevity.

To each their own though. Neither way of going about things is intrinsically better than the other.