From what I've read and heard from other people, the .msp is a Microsoft installer patch used to update the OS with security patches, hotfixes, etc as well as updating some other Microsoft programs. The .msi is a Windows installer used for some Windows updates as well as some other 3rd party program installers.
If a new patch or hotfix comes out, it will scan the computer looking for an existing .msp file to use. If there isn't one it will simply bring along its own installer. Same with the Windows updates. If a .msi is available it will be used, otherwise the update or 3rd party software brings along its own file.
So it should be OK to uninstall .msp and .msi files. They may shave a couple of seconds off of an installation but they shouldn't be critical to anything else. At least this is what I've been led to believe.