> The only extra overhead is of the network, which is something a distributed lock would also have.

Well, There's also the 'overhead' of connection pooling. I put it that way because I've definitely run into the case of a 'hot' key (i.e. imagine hundreds of users that all need to download the same set of data because they are in the same group). Next thing you know your connection pool is getting saturated with these requests.

To your point however, I've also had cases where frankly querying the database is always fast enough (i.e. simple lookup on a table small enough that the DB engine practically always has it in memory anyway) so a cache would just be wasted dev time.