Although I see some good points on both sides, the one thing that I can tell you is that the days of shooting the same girl for multiple clients in the same day are long gone for me. Not only did I find it a complete waste of time, I found that those clients were often the worst to deal with.
Beyond that...I think it's disrespectful, to my loyal clients, to offer to "piggy back" a cheap shoot off of their dime. Risking the loss of a loyal client for a few hundred bucks more is simply a foolish and greedy thing to do.
As for the client the extra stuff is being shot for, delivering a shot out model to them isn't really fair either. They may have a lower budget but they still want decent content for what they can afford. The only exception I can think of for working a model that hard is if you are shooting for a solo site and the "tired" look contributes to the "personality/different looks" factor.
There used to be a shooter here who got caught shooting the same exact sets and models for two clients. He would shoot for client A with Camera A on the right side of the bed and use camera B for client B on the left side. Just jump back and forth as the model stripped.
I guess what I'm getting at is that just because you "can" do something does not mean that you should.
