well your thumb does have to be curved around the bars or the hood to anchor your hand for braking. And climbing. And just generally holding on when the road is rough.

I know it s*cks but I just wouldn't ride. Once that bone is healed, it's healed, and if it heals up with a deformity that you can't live with, they'd have to re-break it to correct it.

I guess it just depends on how much you do with your thumbs (computer keyboarding, doesn't matter so much, especially the left thumb; but if you play certain musical instruments it's probably important that it heal straight) and how much it would bother you cosmetically to have a crooked thumb.