That would imply people would go to those BLs and play the game normally. If anything, they'd just set the controller down only to pick it up to confirm the next match. There's nothing wrong with playing new characters at all, but just sitting there while the other person wails on you or runs the timer isn't fun for the other person. And yeah, you can do all this in Casuals, but for most people it'll be easier to do in BLs. That ease of use changes the notion of "well, I'd never do something like that" to "maybe I could do something like that" for less dedicated people.
It's not that I don't want XP in BLs, but I'd like Capcom to have a system in place so that if I show up to a lobby, I'm not gonna deal with the same shit that sometimes happens in Casuals. I don't even know if that's possible. Maybe more segmented BLs with more options, but the ones that let you get XP require you to press more than one button? I'm not sure.
That's another thing. Matchmaking can take anywhere from 3 minutes to 10 minutes between matches. And you get 3 matches per run. That's if the person hits rematch, which most probably don't.
BL would let me have best of 7. The immediacy and non-reliance on Capcom's servers means I can do this shit faster, all I would need to do is maybe baby it a bit and let it run while I shit around on the internet or something. So while autofiring is probably easier, using BL would be theoretically faster since you aren't beholden to outside factors.