I wouldn't agree they are a waste of time, but I would say they can't be relied upon. I have met a couple of really good ones from reverse bookings. But I tend to ask for a couple of hours, so even if the hourly rate is not that high, the total cash for the meeting is OK, which probably helps. I have also come to accept that if no-one I like replies, I am better off not punting than taking a chance on someone who I have doubts about. That doesn't happen too often, and if I end up losing a couple of credits on an RB which does not go ahead, where is the harm?
My normal RB is:
Outcall
Posted up to a month in advance (and most bids come in the first week when I post it and the last before the meet date)
2 hours or more at something like £75 per hour
Specify no bareback
Explain what you are looking for (ie age, dress size, any specific services).
Most of the time, this leads to a meeting which is at the price I am willing to pay and it turns out fine. Sometimes better, sometimes worse, but I have definitely had a lot more good experiences than bad from using reverse bookings.