Just this last 2 weeks or so I have been asking myself the same question.

Some great fights go the full 12 and you know both guys still have the stamina to go another three and you just don't want it to stop.

On the other hand many fighters would struggle, get too tired and take shots they are unable to react to(defensive wise) due to lack of stamina which highlights the possability of serious injury you mention, but that can happen in a 12 round fight anyway, some fighters have better stamina than others.

I personally can't decide which I favour.