I'm a fan of memcache specifically because ALL it can do is be a cache. No one can come in later and add a distributed queue to it. In-memory caching is also underrated, I agree. Using a hashmap and a minuscule TTL (like 5 seconds) can have huge performance benefits depending on your traffic, and it takes like 5 minutes to code up.
Then I would have to explain „no, we have caching stuff ‚in process’, just use that, our app will use more RAM but that’s what we need„.