I recently finished up migrating a site for a client (WordPress MU) from one host to another. I got an email requesting the migration from someone that said “it was a simple blog with only a few plugins”. When I took my initial look, I counted 30 plugins, and a database with 2093 tables (WordPress MU standard install uses 7 tables). When I asked them about the database size, they were just as shocked. Come to find out – they had a ton of “spam blogs”. After a few hours of weeding out some of the spam blogs, they were able to get the database size down to 1390 tables – which made it slightly more easy to work with.
The lesson here is – if you are going to use WordPress MU – I would consider requiring approval before a blog is created, or at the very least, monitor the blogs on your site. Of course, I would also highly recommend using Akismet for comment spam as well which will also cut down on the general spam level per blog. Also something to note, before you move your site to another server, delete all the caught spam so your database is less bloated – this will save time with the export/import of data.