This depends on how bad the credit is, and if that person will reside with you (occupant or non-occupant co-borrower), or it is parents purchasing with/for a child.
Really bad credit co-borrower is generally not good, however, if he/she improves their credit scores - they could help, if they will reside with you.
Non-resident co-borrowers go under different guidelines, and while they could help some, the whole picture is looked at by the bank (their having to pay their own rent/mortgage/car/student loans etc).
If their own obligations are "too heavy" - they won't help much.
Hope this helps,
Beachfront Realty, Inc.