If anybody in this thread needs help finding a reason to drop a main, think of it like this:
If you're seeking mastery of the game, or at the very least a well-developed gamesense and high-level ability to compete, then I strongly consider focusing on the game through one character. Learn the fundamentals of the game through a consistent set of 'rules' as determined by your character pick, execute on your bread-and-butter skills until they're sharp as a razor through sheer focus and hyperbolic time chamber-like practice, and understand the game through one character's physics, movement, movesets, and so on. If you're the kind of person who's got "PM Mains: Mario, ROB, Jigglypuff, Ivysaur, Charizard, Wolf, Link, Zelda, Bowser, Lucario" in their signature, then this will benefit you.
The first thing you'll need to (and start) realizing, is that you will never have a higher understanding and truly consistent experience with this game through the play of multiple characters. Don't main multiple characters to patch up weak points in your particular matchup without first truly learning how to deal with that matchup with your character. Your level of growth will accelerate once you've begun to focus on the game through one lens. Trust me, attempting to learn this game while switching around between multiple sets of stats, physics, movesets, and various character-specific properties will only stunt your growth and hold you back. Only once you've developed a true sense for the game and a real understanding of fundamentals (which goes waaaaay beyond simply knowing universal and character specific tech) can you begin to move forward and feel truly comfortable through another character.
Look at any top-tier player out there right now, and examine how they got there. It wasn't through use of multiple cast members to make up for poor matchups, no. Those players took years to figure the game out, and years beyond that to develop on that understanding of the game. Once they reached a certain level, these players were able to move on comfortably and effectively to another character. M2K didn't master Marth and Fox at the same time, after all.
Anyway, hope this gives some of the people who end up in this thread a reason to stick to one character. Trust me, if you're in it for the long-haul, for the serious play, then this will help you tremendously.