Securing your WordPress installation is critical, whether on a single-site installation, or on multisite installations. WordPress is the most popular CMS right now, which means that a lot of hackers are targeting it simply due to the large number of users. People aren't always good about keeping their WordPress installations up-to-date, which makes the job of the hacker even easier.
Imagine, if you will, that you're running three blogs, and all are running as single-site WordPress installations. When a security update or other new version of WordPress is released, you'd have to do the upgrade on all three blogs.
Now, if you had those three blogs running off a WordPress multisite installation, you'd just need to do the upgrade once and be done with it. Same goes for updating plugins and themes as well. Do it once on the multisite network and the changes are applied across all network member sites. After updating, you should manually verify that the...