An annoying thing about agencies is having to tell them to fuck off. If an indie arrives - or you travel halfway across town to see her - and she not as perfect as you'd wish, you might still make a connection and feel you are cutting your losses. She's given a bit less maybe in terms of living up to her photo but you estimate she might give a bit more in other areas.
With agencies, any rip-off is institutionalised. She'll probably give you the standard fare, like it or lump it. When you tell her to get lost, you have to deal with the agency and explain she wasn't up to their (ahem . . . ) "usual" standard. Otherwise you'll go on their timewaster list - which may or may not be disadvantage. better agencies and parlours - at least in Scotland - will listen to and act on reasonable complaints. But many won't. They are just the pimp protecting their shortsighted interests. Even many of the best agencies in the country are prey to this type of double-dealing.
With the indie, it's one-to-one. She realises she's stretched the truth with her double-D saggy tits and 'forgot' to mention the stretch marks. But she was pleasant enough, on time, and gave good head. You can negotiate a better future deal for her 'quiet days.' You're dealing with a person, not a corporation.
Fortunately with UKP and modern sources of info like UKP, word gets around. One can learn how to 'play' the agencies and parlours in one's area. But they are still a Plan B compared to a good indie.