If you can afford it pay it, if you can't or don't want to, then don't. It's as simple as that surely?
I don't use agencies, but then I don't need camping passes, so as far as I can tell the only reason to use them is to get your choice of campsite rather than the seemingly random ACO allocation!
Yes the agencies, charge more than the ACO, but we all have a choice! Same as where you buy your groceries from, if you are a cheapskate and suicidal you go to Asda, if you want decent food you go to Waitrose
At the end of the day those that use Agencies accept the price they pay, end of story!
I managed to find 20 KN tickets for us through an agency..... £4000.
Thats Harrods money....