Chris, this request is already on the wish list.  It is a feature we've had for as long as I can remember on Yahoo Groups.  If Mark can add a "mandatory" option, then, folks can either require a comment, or not.  For my groups back on Yahoo Groups, our group description stated that a membership request must have a comment included that relates to the purpose  function of our group.  If someone tried to submit a "filler" comment, like "hi", their request was rejected.  Worked fine for years on Yahoo Groups, don't see why this would have a "real problem".  In fact, Mark can even make a comments section visible or hidden for list mods/owners if he wished.