I can see not replying if a reference comes back bad - you do NOT ever want to tell a client "Oh so and so said you were rough with her" as that can have safety repercussions for the other provider. Even writing and saying "I checked with a couple of your okays and they said X, Y, Z" can be - as the guy can usually figured out who would say what. In that case, no reply is appropriate.
I try to reply to everything I get, even if I'm not in that town that day, unavailable, or whatever the case is. I try to simply reply and say "I am not available on X time on Y date in Z city, my apologies. I hope our paths cross in the future!"
I've also seen autoresponders that say something like "I am unavailable X-X dates, if your request is for one of those days please accept my apology in advance for not replying".
I think silence/crickets/suspense though is unprofessional.