I think this makes sense.

As an end-user, I am not just looking for a system that has fewer bugs. I am looking for a system that does not add NEW bugs not present in the system I am already using. Because those are the ones that are going to cause me problems (and possibly big ones). It is too easy to make a "quick fix" that has unintended implications. The later in the RC cycle these happen, the less likely they are to be caught and fixed or excluded.

The beginning of the merge window is where you dump your backlog of fixes that you want to get into the kernel. RC5 should be fixing bugs added in RC1 or later (or things with big security implications).