Choirs and rating agencies: yes, they have something in common

It occurred to me that a choir might be a good metaphor for rating agencies;  at least to some extent.

Recall that certain rating agencies got blasted because they frequently awarded their highest ratings to pools of subprime mortgages, which on their own would have been rated much lower. Was the theory that by pooling them somehow the risks would cancel out?

Well,what if we took a group of mediocre (or poor) singers (some who were tone deaf, none who could read music, some who sang through their nose, some who couldn't enunciate) and pooled them together into a choir. Do you think that they'd be an A-list type choir once they were together (their poor qualities canceling out)? I guess this can be considered a rhetorical question, as we all know the answer.

And so, if a group of  poor singers together doesn't produce a highly rated singing group, why would a group of subprime mortgages?