I'm no expert, but I like to look at what successful people do. When I look at the results from EVO 2014 in almost every game the people in the top, whose names make the results on shoryuken, have one main character. (Or in a game that requires two characters, like TTT2, they have two).
http://shoryuken.com/2014/07/13/evo...argest-fighting-game-tournament-in-the-world/
Like Awesmic said, there are those very rare folks that can get good results with multiple characters, who work hard and have natural talent, but they are the rare exception, not the rule. If you're one of those, you'd probably know by now.
To put it into perspective, I have two particular PSN friends that are very much into NRS fighting games. They go to majors, they go to EVO every year, and they also have a single main. A different main for each, but they both subscribe to the one main philosophy. They pretty much play nothing but fighting games, and pretty much nothing but NRS fighting games, and between EVO 2013 and EVO 2014, they played pretty much nothing but Injustice. One of them was on virtually every day and I saw him playing Injustice 95% of the time, hours at a time, and MK9 the other 5%. Literally there is only one time I can remember that I saw him play a different game in the entire year. Every time too, all those days I saw him on it was always the same character, working that main, learning the stages, the match-ups, every nuance.
Both made Top 64, but neither made Top 32.
So hours a day, every day, playing the same character in the same game, and even Top 32 wasn't guaranteed. That's the level of competition you are dealing with in the FGC.