That is not necessarily true. Some refurbs are "buyer's remorse" returns. Even if they did have something wrong at one time (maybe a bad hard drive or whatever), Apple returns them to like-new condition before selling them on the refurb store. If anything, a refurb has been looked over and tested MORE than a new machine that comes off the production line. Refurbs get the same warranty as new machines as well.
I've gotten nothing but refurbs from my 2nd mac on forward - never had a problem with any of them and I credit it to the repair and 2nd round of testing they get.