I usually play three characters in any given fighting game with any kind of consistency (except Blazblue, where I didn't even have a main until CSEX, so I literally played with everybody throughout each iteration to keep myself from dropping it, until CP). No real rhyme or reason ("things come in threes?") except variety. Keeps things fresh. My mains are usually good enough characters on their own to not warrant secondaries in a competitive sense. Hysterically, it's not that I just pick 'em because they're good; the decent characters usually are the ones that jive with how I play a given game. The other chars I play on top of them are usually icing on the cake.
In Smash 4, however, it's going to be kinda important that I have a secondary of some kind. D3 alone isn't going to cut it. His bad MUs are bad, his weaknesses are glaring, and his gameplan/ceiling is low in potential. I want to play through these traits despite how I or my peers feel about them, of course, but in my heart of hearts I know I need a contingency plan or two for competition. That's why when I fully figure out this character, when I've exhausted everything, I'll put in for a secondary to cover his flaws. Mewtwo is probably a safe bet for this, idk yet.
Smooth Criminal