as to the original question... to me it just depends how you want your classes balanced.
if he was going to be zero help for this years team, and you like the idea of him being on the team 5 years from now,, then do it.
i guess the biggest tradeoff is he wont be quite as good in his 4th year as he would be if you didnt redshirt him.
hate to agree with mikey, but probably depends how much potential there is.
the problem with tissot idea , which , if it worked might allow him to improve slightly more this season, is the RS is already on... to do what tissot is talking about you have to take it off, and then reapply it at end of season and i think sometimes (or maybe all the time now?) the WE lost is worse on second try.