There are plenty of game modes in Arcade. Why choose the only mode where you have no say in who you'll be if you want to choose who you'll be?
I don't want to choose who I'll be, just choose to *not* be something bad. For example, last night I spawned as Zen and my team also had a Lucio and Mercy (and a Sym). Despite being a Zen player, I believed so many supports are dubious in a team composition, so I suicided. I came back as Pharah, which we already had one of but since we were lacking in firepower and we had at least a tank (and I didn't want to risk a worse roll by suiciding again when I wouldn't have time for a 3rd respawn before the match started), I kept on it. Incidentally, it turned out to be a good choice as I stayed alive the whole round and we full held the point.
As for why I play this game mode: It's the most consistent. I'm awful at 1v1, the long waits of 'nothing' in 3v3 when you're dead annoy me, and the brawl playlist has allot of very unfun brawls. That leaves just Mystery Heroes, which, by the way, I
do have fun with. The 5th mode, whatever that may be, I sometimes make my primary mode of play. I played Oasis almost exclusively when it was in the Arcade, but I don't plan on playing CTR again, so that leaves me with just Mystery Heroes for now to farm my weekly boxes.