If there's every a security bug in a library, you're left vulnerable until the patch is issued, and then every individual maintainer patches their binaries, and you get around to installing them. With dynamic linking, you just have to patch the one lib package and you're done.
The other nice thing about packaging systems is that you can choose how fast you want things to be updated. If you're having trying to install Gentoo on a toaster oven, things will break; if you install Debian stable, you'll never have to worry about an update again.
