Some agencies/schedulers can be pushy when dealing with them. I prefer being able to communicate directly with independents.

I get from an agency is that it virtually eliminates last-minute cancellations.  When I go with an agency, if the girl I booked has to same-day cancel on me for whatever reason, its kind of late to start over if I'm dealing directly with the lady, but an agency will usually be able to provide a substitute on short notice, so I have not cleared my schedule for nothing.  For me, scheduling providers around my work commitments is the most challenging part, so it really sucks when someone has to cancel the same day as the appointment.  Getting someone else on a few hours notice is very difficult.  The agency knows me and can just recommend a replacement.  

With an independent, it's more likely that she's getting 100% of the donation, and that's important to me. I'd rather pad the purse of a woman who's paying rent or mortgage, buying groceries, keeping the electricity and gas flowing, and socking some away in her savings than for my money to go to an agency.
Of course, I have absolutely zero way of knowing if an "independent" truly works for an agency UTR, has a pimp, is being trafficked, is being controlled by someone, has to hand the money over to someone somehow, or is a druggie, an alky, a gambler, a shopaholic or whatever. I prefer to think that my donation is her money.
Years ago as a single man, and long before my hobbying days, I went on a first date with a single mother. I stopped by her apartment to pick her up, She had me step in while she finished getting ready. I noticed that her cupboards were bare and her children  threadbare. Instead of spending $30 or $40 on a date, I took her to the nearest supermarket and told her to fill up a shopping cart.  I paid for it, we went back to her place where she cooked a meal and she had full shelves for the rest of the week. That always stuck in the back of my mind, and now that I'm in this life, it's still there to a large extent.