I don't know what the prices are in your area, but in Scotland the agencies don't appear to charge a premium, which is great for punters, but I struggle to see why the girls would hand over a cut of their earnings for what appears to be very little in return, when they could just create a profile on AW and get customers at very little cost.
If the agency is well respected then no reason not to use them, might be that they provide an incall place, or the girl is a bit flakey and needs someone to do all the admin for them.
However there are a couple of agencies in Glasgow that give me a dodgy vibe, so watch out for that too. Mostly advertising on VS, prices are suspicious, photos questionable and advertised in multiple locations. I've not seen any of the girls reviewed, so assuming it's a scam of some sort.