It seems to me that most applications would be better served by motors than muscle wires (especially anything of any size beyond teeny-tiny moves) and especially in mobile/battery applications. Mainly because motors are usually at last 50% efficientes and I think muscle wires are somewhere around 3%. Of course there's also the issue of lack of displacement and dependency on ambient temperatures which is "absent" in motors.
It would seem that to use muscle wire, you need to have a very specific small scale application (like the dust sensor on the mars rover or finger sized robots). But I'm talking here completely from a mobile/battery powered perspective- I think I calculated last time that to get the forces and displacement I required would have take 120V if I used long thin wire with pullets or 60A if I used shorter thicker wire with less pulleys.
However, if you have something that's tethered to a wall power source and works in something like water, you can get very interesting results like the robot lobster. It works in water and is tethered so thicker or longer wire can be used to get high forces while maintaining fast response time (due to the cooling of the water).
So things like arms, fingers, sensor pan-tilts, wheels, legs, and the vast vast majority of actuators on robots would have better performance, efficiency, cost, and simplicity than muscle wires. THe exception of course is on the legs of very very small finger-sized robots- except these robots tend not to be able to think or anything- just blindly walk forward for the most part. The thing muscle wire might find a use on is on a complex robot with lots of little things, at which point a tiny silent actuator for a minor part on the robot might come in handy. The thing that most immediately comes to mind (without the use of long strands of wires and pulleys which would result in very high currents being required, whereas thicker wire would seriously reduce response time), is something like a small light weight sensor to have movement like an eyeball in an eye socket- something like a diode-sized photodetector or a tactile antenna. Basically an application where "force" is a criteria is probably too much for most muscle wire methods.
Two silently moving antenna like a cockroach is damned cool though.