It's still much better than the alternative, which is not having any kind of way to reproduce race conditions :-) QEMU's a pretty complex multithreaded program and my experience with chaos mode has been that it's quite good at tripping up the races (and you only need to catch the race once).
It's not limited to any era - it's a general-purpose tool that can perfectly reproduce bugs, even most race conditions. The lack of multi-threading will mostly just slow things down. Being able to perfectly reproduce a bug is immensely valuable.
I didn't say anything about the usefulness of rr other than in the context of concurrency bugs so I don't quite understand a defending attitude of yours.
