My tuppence, for what little it's worth.
I did both, JOC immediately after the MCC. What I found is that the transition from piston to Jet proved very hard work for the first couple of MCC sessions, such that both me and my partner felt we got relatively little out of it. The remaining sessions were markedly improved and very enjoyable. However, the JOC was better still due to better experience in the sim and understanding of the response of the [aircraft]. We felt the JOC gave a good view of what we could expect in a "typical" day on the line but more importantly gave some vital extra experience in handling the a/c.
Thus, I would say it's validity depends on your wallet, as with all things training. If you can afford it, I think it's very useful, in the same way that if you could afford to do an extra 10 hours before your IR most would think that worthwhile. If you can't afford it I don't think you're at a huge disadvantage, but if you can, it's worthwhile.