A few times I have taken the commission to obtain SL or commander roles, because the ones playing them were not using their class (not throwing smoke, going for objectives, placing arty), and they refused to do so even after being asked to several times.
But other than that (which is not often, because I usually play SL myself, so it is unnecessary) I also always decline them.
Just yesterday I was playing SL on Apartments, being no. 3 on the list regarding team points (and as usual most sure to be no. 1 by the time the round was over), when suddenly I had the dreaded "mouse pointer on screen freeze", ESC cures this, but I know it is a sign that someone has stolen my role, and sure enough as soon as I died I was taken to the choose role screen and no SL roles free, only riflemen and marksman.
That was my last game that evening.