I can answer that one.
Some of us just like variety. I play about 70% of the cast because I have fun doing so. If I get tired of playing one character, I can either default back to Izayoi, or play one of a dozen or so others. It's just fun for me I guess. Usually if I get asked to stay on one character I have no problem with that. Hell, I spent 2 hours once playing Valk so one of my friends could get some matchup experience. Just ask the guy to pick one character, you never know, he might just do it.