I don't think I'd use it even if Redis was a bottleneck [1]. With a performance increase of around 11% for SET/GET commands, I don't think there's that much to gain from running redis without an underlying OS. There's quite a bit to lose, too, because if you think about it, the implications here are that you aren't going to (ever) be able to make your data persist. Redis is a better fit for data that doesn't necessarily require persistence anyway, but putting this all on a scale, 11% don't win me over.

That being said (which is specifically about Redis on Xen), though, I think being able to run other server software without an underlying OS is pretty damn neat.

[1] Redis already scales nicely and with the upcoming Redis Cluster it's going to be trivial to run a fault-tolerant scaleable environment.