Newbie - FAQ
Of course, P411 requires either references or traditional screening (e.g., full real name, employment, etc.). Once you have references? The primary benefit, in my opinion, is convenience. Both yours and the ladies'.
First, instead of emailing another lady you want to see and providing the two references -- the names, contact info, and how the reference lady will remember you (and some may forget fairly soon) -- you can just send her your P411 code. (Even better, as Gina recommends, you can email her or request an appointment through P411 rather than sending directly from your email account. That way she knows it's you.) You don't have to hunt for contact info, try to remember when you last saw the reference lady and what you said or did that might remind her of you (important if it's been awhile since you've seen the reference lady). It's quick and easy.
Second, once the new lady gets that P411 code or message/appointment request through P411, she can "screen" you very quickly. Some ladies want to go beyond the "OK" on P411 by actually contacting some of the ladies you've seen, since their definitions of "OK" may differ and she can also get some insight into what you're like as a client. But most ladies, from what I've seen, will accept you as screened as long as you have a couple of OK's, not too old, from ladies whose names she recognizes. Literally a minute in many cases.
That makes the screening process much more convenient for the lady but also benefits you. Without P411, you provide the references, she emails or phones them, they don't respond right away, and she has to wait until they get back to her. Which could be an hour or could be two days. If you're trying to schedule with a new lady on short notice, your chances of being screened in time are much better with P411 than with the regular reference system.
Those are the obvious advantages, anyway. Whether they're worth $69/year is up to you. It is for some people, not for others.
As far as ladies who say that two current references are "non-negotiable" . . . if she's also on P411, she almost certainly considers an "OK" on P411 (if it's current) as equivalent to a reference. That is, after all, what it really is. I've never run across a lady who was on P411 but wouldn't accept that for screening. Many of them do require more than the Basic membership, that is, that you have a few OK's on your account. But they usually disclose that in their P411 profile under "Willing to See": Basic, Basic + 1, Basic + 2, whatever. I would guess that somewhere between 20% - 40% of the ladies on P411 will accept it as verification even without an OK. (I think that's risky on their part and I wouldn't recommend it, but that's my guess.) That's solely a wild-ass guess. You might try contacting P411 to ask if they have any statistics about that. Of course, what matters is not the averages for all the ladies on P411, but what is accepted by the ladies you want to see. 
Good luck.j