Since everything processes there, "bad neighborhoods" could arise; the sort where every resident has a ton of poorly-written scripts and a ton of object prims floating around... That area would be noticeably slower than an area on the next server where there might be an SL HOA, mandating certain behaviors.
See the screeshots in "Second Life uses MySQL" where you see tons of spheres flying around: I haven't been on in years, but I remember there were various devices people made to spawn thousands of objects at once and crash servers (or "sims" as they are called).
You get higher efficiency as most of the data is in a locus around one sim - so it stays on one server and you don't need to move data about - check data, reorder data, etc.
On the other hand, you get lower overall utilisation - as you might have one server maxed out, with ten sitting idle right next door.
So interesting they've taken this approach - it's got the significant advantage of being more simple and robust than other approaches.
http://secondlife.com/whatis/economy-graphs.php
I'm not saying it definitely has been.