For an interesting example of a haptic musical interface, check out the University of York's Cymatic: http://www-users.york.ac.uk/~smr12/main.htm
This gives me a better idea. It is not by chance that they focus on physical modeling. There are many ways to make sounds with a computer where haptic interfaces makes much less sense.
During a recent improvisation workshop, some people complained that I was totally inexpressive while playing, even if I was able to make "gestural" sounds and make use of the whole dynamic range which is more extreme than the rest of the players (instrumental).
I noticed that my teacher can (on occasion) use an artificial gestuality (that is, not justified by strict interaction with the device). For me this is not necessarily a problem, this split being a part of the nature of playing electronic devices.
Regards, Maurizio Umberto Puxeddu.