Well, I didn't even know about mirrors.linode.com. Mine were still the ubuntu default servers.

I guess apt-get from one of Linode's mirrors saves bandwidth? Or is it just more polite?

It probably ensures that you get the Linode-customized flavors of packages where such exist, so that, for example, you don't inadvertently upgrade your kernel to a build without the ability to mount Linode disks.

Linode can't modify packages; they're signed by the upstream distro. (Unless Linode added a key of their own to your apt keychain (apt-key list), but I've never seen that.)

It's also meant to help you and Linlde save bandwidth!