Speed and Simplicity
Until recently, Memcached would still obliterate Redis when it came to read / write performance, it's only in early-2015 that Redis finally caught up (I haven't tested it recently, but looking at the architecture changes in Redis, it should be on par with Memcached).
If you're only looking for caching, I would also have a look at Hazelcast, it uses the same protocol as Memcached. If you're already using Redis, keep on using it.
We're still using Memcahced to cache session tokens.