Keep it ... it will serve as a back up lens ... also, as you said that by selling it "you won't make much on it anyway" ... besides if/when you want to do some video in low light (at that focal length) it will come in really handy. Over a period of 6 years I had three Canon 50 f/1.8 lenses, the only reason I don't have any of them now is coz I gave them away as a "free gift bundle" to "enhance" the sale of my Canon 400D, 500D & 7D. So keep it, if for nothing else one day it will come in handy when you want to sell some of your other gear.+1
Might also be a good lens to take to the beach.
I still have my 50D & 28-135 kit lens. Couldn't get that much for it (not enough for a small L lens) and thought if I took a trip it would be a good backup body just in case.