I used to think exactly like you do; After having several crap punts through Adultwork I would turn to escorts from Bijoux, Hamiltons, or Allure and have a relatively good punts, even if they were expensive.
I'm not so sure about them these days though. I still use them from time to time (maybe once or twice a year) but I've not had that many great experiences with ladies through these agencies of recent. No particular bad experiences, but rather mediocre / average punts and nothing that justifies paying the premium price for them. Also, most of the escorts advertised on these agencies also advertise for a lot less independently on Adultwork. It can be a bit difficult tracking them down but they are certainly there if you search long and hard enough for them (I suspect they are not getting much work from these agencies at the premium prices they advertise for).
I have also improved and fine tuned my search and selection screening on Adultwork, so I have far better punts than before. I have even punted with ladies who advertised, or used to advertise, through these agencies and paid a lot less for them through Adultwork. The service you receive from them is no different.
I still think these agencies (Hamiltons / Bijoux / Allure) are far better than other London based EE agencies like Hot Collection / Diva / Barracuda, etc. But, like how I gave up on those agencies many years ago, I am at that point where I no longer need to use these agencies.