It is plausible that you usually
don't have any need to manage the database. However, databases don't completely
manage themselves yet, and even automated systems may need tuning.
MySQL also offers a variety of storage backends
(MyISAM, InnoDB, merge, memory, BDB, cvs, NDB, ...), and in some usage patterns, it pays to switch to something different, especially in multi-user environments.
My guess is that database server management is an NP hard or NP complete problem, so I find it unlikely that there's "no extra management overhead" in the long run, even if Adobe were clever.
But for a few users, you are likely to be essentially correct.
What I was meaning is that the care and feeding of Version Cue only has to be done one, even if Lightroom traffic is added. If Lightroom's database could be handled by Version Cue, I'd expect Lightroom's database to be a separate database on the Version Cue MySQL server, utilising the existing user management provisions in Version Cue.
I run MySQL on my server here, so I'm familiar with the different storage backends. Unless I've missed something, Version Cue doesn't expose the choice of storage engine to the user - I'm 99% certain it uses MyISAM for everything, though maybe InnoDB makes more sense (because of its transactional nature).