Because he would still be legally responsible for the mortgage should you default. If he would get into financial problems and someone gets a judgement against him, they could put a lien on the house that would have to be settled/paid before the house could be sold, or the same for you. All a quit claim does is say that he is giving up his interest in the house, but there are still other legal ramifications possibly involved.
Since you are going to re-fi, you have no worries. Call the clerk's office at the courthouse. Find out where to get a blank quit claim deed. Both of you fill it out, have it notorized, file it at the clerk's office and re-fi as soon as possible. Best of luck to you.