The thing is, let's say you have 500 user blogs. Each of those have an average of ~300 posts and pages per blog.
That's some 150,000 posts/pages total. Funny as it sounds, it's faster for mysql to look up something in 1 of 500 tables with ~300 entries, than it is for it to sort through 150,000 rows in a single table. At least according to what I read in the mysql manual yesterday in regards to optimization.
Now, also to consider is scaling purposes. If you get to a point where you need to break up the user tables into multiple db's, having a table like you're describing would hinder that process.
So yes, less tables are easier to manage from an admin side since there is less to keep track of, but at the same time, it's more abusive on the server as well.
It's been a topic of a lot of debate here, and you can see what's come of it. (The current db structure).
The question that "should" be in circulation, is whether a single global db with the global db tables, and then putting each user in their own db would be more efficient. Of course, then you're managing connections to 100's or 1000's of db's too. So it comes down to working with a ton of data, and what's better.
If each user had their own db for their tables, that would be easier to manage, and easier to scale than separate tables. Then separate tables are easier to scale that single global tables, but the single global tables are easier to manage then 1000's of tables.
A vicious cycle indeed.