For me, it depends. There are only like two or three characters that I play in SCV. As for the rest of the cast, why should I give a shit about them lol
Master of one. I kinda feel like the characters I play as kinda represent my personality. Playing Pyrrha just feels awkward enough since she's apologetic and a girl xD but that's just me
And I guess it's fun to have a challenge, like the nm vs Pyrrha MU which is like 7:3
While I'm in the camp of people who advocate players having strong mains, I think you should at least be somewhat familiar with every character to be fully prepared for every matchup.