Plugins are a "Top Down" roll out and should be. (Same reasons the Theme editor/installer is Network Admin only).
Only the Network Admin(s) can install plugins - gives you the ability to test/activate them site-wide first. You would NOT want an untrusted user testing plugins on one site that will bring down the entire network. Can you imagine what a ne'er-do-well user would do with a plugin specifically designed to bypass WordPress kses?
Anyway, if you trust a user to install plugins, you may as well give then the keys to the whole shebang. Make them a Network Admin, too.