The other day at a steakhouse the waiter asked me if I wanted Ice in my Martini. I told him I didn't even wanted it shaken more than 3 or 4 times.
IMO, since a Martini is meant to be consumed ice cold, there's nothing wrong with one on the rocks when you're in the mood to sit with your drink for a while. Otherwise, you have only a few minutes to quaff the thing until it starts warming up (not that this is a bad thing ). In these situations, I'll take a somewhat watery Martini on the rocks instead of a tepid one straight up.