Ability to repay and willingness or often 'responsibility' to repay can be two different things. If they have a trashy credit score, but can well afford their debt obligations, then adding a strong guarantor/co-signor would be a S&S issue, IMMHO.
True story: We actually have a customer that makes a really good salary, can well afford his debt, but CS is in the low 400's. His honest answer for this was...he just really likes to spend money.
So, I guess he's so busy spending, that paying the bills on time is just too much time away from shopping and spending?