Defoe is on lower wages than Adebayor. Of this I am sure. Ergo, keeping him as a back-up striker to Soldado makes sense, given that paying Ade 100k a week to sit on the bench is madness for a club of our financial stature. He is also old: old enough to hopefully accept that his days of being an undisputed starter are over. Ergo, keeping a contented, relatively cheap poacher as a back-up makes sense, even if that striker is less valuable tactically than a more rounded player like Ade.
Plus, sentiment comes into it as well. I would like him to retire here, if he wants to. I think he's earned it, being our fifth highest scorer of all time and all.