The set bonuses would have to be pretty significant considering you would have to give up your exotic slot to wear a full set. For instance, the bonus would have to be pretty damned good to get me to use a legendary helmet over Saint 14 on my Titan. The problem is then you get into the issue of whether set bonuses are OP because they trump the bonuses from using exotics. It would certainly add an element of diversity to the game, but I'm not certain it will work out well.