He's probably kept around because other than De Gea he is the only one remaining with PL/CL medals, and he is required to wear them on his track suit at every training session with the kids.
The fact that four managers have renewed his contract must indicate he has a value unseen to the average fan, perhaps he could (should he wish too) give away many secrets/skeletons from the dressing room.
Whatever, when I have seen him play (not that much recently) he always gave 100% and is fearless as a lion, or maybe that should be a kamikaze pilot!