I think we have all been there and done that.  Not a bad rule, but then I think back to some of my more successful interludes and they were ladies that behaved the same, although didn't stand me up after all.  I recall one ATF's explanation that too many would-be clients make advance appts and don't show.  Many of these ladies only book one or two clients a day and a no-show costs them, especially if they have turned down others.  Many of these ladies have this booking policy for 1st time clients and then after that trust them enough to make advance appts.

If its a known reputable agency (although many of us prefer the independent types), you should be assured of having a real appt, but many will still require that you call back to confirm you are on your way to avoid down-time.  I have rarely been stung this way by well known agencies.  

On the other hand, if you have planned ahead and made an appt with a lady that requires you call 30-60 minutes ahead of of the appointed time to "re-confirm", how is it that you know you really have an appt.  More likely, she has double booked and the one that sounds most interesting (or profitable) has the real appt.  

And I also know that some will use this response to put-off clients that they didn't feel comfortable with during that first phone discussion.  Perhaps the client prospect used language that made them nervous or asked indiscreet questions.

My advice - 1) when doing the first-time booking, don't ask any questions regarding fees or services.  do your homework ahead of time regarding this.  some don't advertise their rates, so discreetly ask for this with a simple inquiry - "can you tell me what's required to book an hour or two with you"  Its ok to ask whether incall or outcall, or availability type questions.  Just stay away from language or even inuendo that even remotely suggests sex for money.  2) be sure to tell her that you are "definitely" keeping the appt and please keep the time open for me.  Be sure to call when you said you would.  3) be prepared to be disappointed from time to time.  Its going to happen if you also enjoy the hunt for new undiscovered talent. 4) if you prefer agencies, develop some loyalty to one or two of the classier local ones. regular clients are treated with respect and will get preference when new girls come on the scene.  5) listen to the tone of her voice and does she really sound interested or are you just hearing a standard response.