I've been doing this for a very, very long time and interestingly your suggestion often works exactly the opposite to what you suggest.
While I have had a couple of girls offer a frequent flyer discount, I have had a number of others tell me that their "special" doesn't apply to regulars just to newbies. The logic sounds counterintuitive but is similar to what a lot of large companies do - trying getting that "special" from your cable, insurance, cellphone company if you are an existing customer versus a new one.
So, not unusual but nevertheless not conducive to building long term loyalty.
As others have said, variety is the spice of the hobby, but I have been loyal to a number of girls that offer a perk for continuing to see them. Certainly wish more would, but I don't think the fundamentals of economics and marketing resonate with many girls.
Let the flaming begin 
Posted By: SoapMactavish
There are some providers that offer different rates for "loyal" returning clients versus new/random clients.
And then there's other providers that don't. I always wonder why this is so, from a provider's perspective.
Definition of returning client - 3+ visits (it's clear this gent will be a reg)
I can understand why these "special rates" exist:
1. create long lasting client base
2. show client appreciation
3. Business strategy: encourage more frequent visits (thus a provider may make more money with that client in the long run, rather than charging more and seeing that person less).
However, playing Devil's advocate, if a provider does not offer these "special" rates does that mean that provider hasn't thought about or doesn't care about the advantages I listed above? I would imagine almost all providers have thought about it but yet 50%+ of providers don't, so there must be some good/strong reasons.
So I wonder, what are the reasons some providers have special rates for loyal regulars and some don't?