Why wouldn't you? As I understand it, Redis is a superset of memcached (since you can turn disk writes off) and comparable in speed. I, at least, use it as a memcached replacement for storing sessions with the added benefit of not having them all expire if I need to restart the server at some point.

Basically, I exactly agree with the GP's use cases. ACID for things I want to keep, redis for small things that need to be fast (sessions, stats counters, etc).