Generally, I would recommend not to categorize memcached as a database - there are existing clients (console, rhc) that treat cartridges with the 'database' category as special. It might be better to use something like "cache" or "cache-server" or "caching" as a category (and then we can add that to redis as well).
We are trying to change this slightly so that environment variables can be generically published, rather than everything having to pretend it's a database in order to expose user/password/url. Be aware in the future it may be desirable to use the "publish an env variable to the app" type rather than the "publish DB connection info" endpoint. There's a story to do it, but I can't find it in the runtime board right now.