If the Background Vocals sound good even with bleeding, why do over dub?

Hi there.
I'm not sure I perfectly understand the question,
But for sure I will say this:
I love bleed in certain things.
I doubled the backing vocals based on the original track having super thick vocals in the hook with a zillion layers... so I had both girls song twice only due to time constraints. I would have done it 3 more times if I could have.
Also the amount of compression I knew I would wind up with on the main vocal made me decide to put the mic out in the main live room for doubling.
Bleed can be great.
Humanity matters.
I told the horn guys to leave the booth door open a little of it made it more fun to record. They did and it sounded great on their part.
There is barely any drum in the mic because horns are loud.... but there is bleed, and I enjoy that fact.

That's why IMO, you got to used the best mic possible, so the bleed from theses mic will be
a nice one......