Most engines will allow you to tweak the settings, and some (like chessmaster, shredder and I think Fritz) will let you select certain "personalities" but why would you want to play against something less than their best settings? Anybody can defend against unsound attacks and gambits. Don't you want to practice against best play? So when you get to an actual tournament or other type of human opponent you will be prepared?
The thing is, I see four kinds of games. Orthodox human games, which means conventional but with blunders; unorthodox human games, which means unconventional but with blunders; orthodox computer games, which means conventional without blunders; and unorthodox computer games, which is what I want to play against. An unorthodox opponent who doesn't blunder.
Now, although I'd like to get better at chess, it's still more important to have fun, and I find playing against opponents who don't blunder but who aren't very conventional either fun.
I don't suppose there's any chess engines out there that can play unconventionlly? I honestly don't care how good it is, because chances are it can still beat me if it never blunders.
When I say unconventionally, I mean it tries gimmicks and risky gambits ;D . I need to practice myself against strong unorthodox players.