Like you I occasionally top because my Master likes me to. Mr P sometimes just likes to loose the control for a bit and he also has a masochistic side as well as sadistic so bottoming to someone for a play session gives him a way to get some pain. This has only been done over the phone and cam so far but we have talked about doing this in person some time.
I'm guessing you don't domme your Master, that its only others but I can relate to the being putty in his hands. I have no interest anymore in topping anyone else I find it boring and not much of a turn on. But if I know that he is feeling a bit masochistic I will quite happily take over for a little bit. Admittedly not very well
I do get turned on by topping him because I can see the pleasure he gets from it and I guess that appeals to the sub in me, knowing I've pleased him. These little sessions of me topping are dictated by his mood so I won't just randomly take over or anything.
I personally see the times when I top as another way to please him and to be honest if giving him some pain once in a while does that then I will quite happily do it. Anything to keep Mr P happy, so like you say putty. I do something for him that I'm not normally drawn to because he wants it.
I know my topping is slightly different from yours but both at the Doms will