I can think of a few reasons why the recruiter is doing this.
- To find out if he should bother connecting you to other clients at all, or forget about you.
- To find out if his ways of estimating a candidate based on resume are wrong and need to be improved. He needs this in order to not present unsuitable candidates to his clients, which at best just wastes everyone's time; at worst, clients could decide to not work with him any more.
From your side, the first question you need to answer (to yourself, before talking to him) is what's the reason why the interview failed.
Was it you (as in, lack of skill and knowledge)? Was it on interviewer/potential employer side (as in, unrealistic expectations)?
Or a communication breakdown somewhere?
If you aren't sure, you could ask a third party - some other senior in your profession - who could give you his (hopefully, unbiased) opinion.
Whatever it is, once you find it, it will form the basis of your side of the conversation.
Should you talk to him at all? Well, if you don't, chances are, he will not work with you any more. This may or may not be bad.
If you do, if you think you were right, explain why. If you think you were wrong, come out clear.
Sometimes both sides are wrong - a candidate thinks that with some basic knowledge to start with, within a few months from his start on the new job he can and will pick up anything he needs; while on the other side, the potential employer is under impression that the candidate they're about to interview already has highly specific and detailed experience.
Also, use this conversation as a learning opportunity. Find out what worked and why, and also what didn't work and why. Find out what were the expectations, and how you can ensure, in the future, that the expectations are realistic.