The problem with the "signature" model is nearly every pro drummer uses different gear for different situations.
We all know that most of the top pros go into the studio (be it a session or their own band) with numerous snare drums and cymbals, and use what's best for that song/session/room/whatever.
So it's always a little weird when you see some of these signature products being advertised, because you know these guys aren't using this snare drum or ride on every single session they do. It's just not practical for them, and thus, to me, is not practical or realistic to be marketed that way.
One of the few exceptions is perhaps Neil Peart, who really does use his Paragon cymbals with Rush, but then again, he rarely plays outside of Rush.
The other aspect of all this is ego. Drummer A develops a "signature" product, and drummer B says "hey, if he gets one, I want one too". Or if Drummer A is with one company, and another company comes along and says "hey, if you switch your endorsement to us, we'll let you have a signature product.."
One more aspect is simply keeping your name in the spot light for as long as you can. Most pro drummers know a career can be fleeting. A hit band one day is not necessarily a hit band the next day. Session work is not the same as it was. There is always the next hot young drummer ready to come along and do the next session/tour/band that would have gone to you.
So I think some drummers see the signature thing as a way to keep their name in the public eye just a bit longer, or at least to take advantage of the exposure while they have the chance. Just pick a Modern Drummer issue from 5 or 10 or 15 years ago and see how many guys were name players who might be better listed as a "what ever happened to" right now.
But yeah, if I were a star drummer and someone made me a signature product, I'd feel like I had to use it as much as possible, and I'd feel really guilty if I was ever seen without it.