Not being a MW developer, nor nearly educated enough with respect to
database programming (I'm learning slowly but surely!), I obviously can't
speak to their intentions nor any possible technical reason for this
particular setup.
I will, however, echo the comments of Emufarmers in saying that the features
of Mediawiki were designed with the intent of serving Wikimedia projects.
With that in mind, it's a safe bet there is some further rationale in there
somewhere.
Hopefully somebody with more insight might share with us.
Dave Pace