The worth of making that change depends, at least in my mind, on how often you find yourself wishing you had a smaller gear(s). How long have you been riding and what kind of climbing do you do?
Like Jenn, I'm a little surprised that you LBS thinks it needs to change out the r/d for at least a slightly wider cassette. I would think it could accomodate at least a 12-27/28. Of course, that comes with a tradeoff in that you will have eveń wider jumps between some gears.
Live with intention. Walk to the edge. Listen hard. Practice wellness. Play with abandon. Laugh. Choose with no regret. Continue to learn. Appreciate your friends. Do what you love. Live as if this is all there is.
--Mary Anne Radmacher