First off Welcome!
To answer your question, yes there is ton's of reasons not to do it, but many of them may not make sense to you at first.
What you first need to figure out is if you really want to play fighting games (8 way 6 buttons and horizontal monitor) on the same machine that you play vertical classics like Pac Man and Donkey Kong (4 Way usually no more than 2 buttons and vertical monitor)
If the answer is yes, then a four way can be added onto a panel tastefully, just make sure you check out the control panel thread for ideas of how to incorporate it so that its A) not in the way of other controls and B) doesn't look retarded.