I'd be happy to have him, but isn't he ultra-liberal?
The point is having his vote on procedural matters and such.
Procedural matters are just that - procedural. Whatever party is carrying out the procedures usually doesn't matter. That's why if you're a GOP House incumbent getting primaried, your opponent might say something like, "John Smith voted with Nancy Pelosi more than half the time! ERMAHGERD!!!" which is technically true because of how many procedural votes are basically parliamentary office work.
Do you really think Angus King is going to vote for a Senate version of the Ryan Budget? Do you really think he'd be the 41st vote to sustain a GOP filibuster of some obscure agency nominee because the fundie Republicans can't stomach confirming a pro-choice individual to be the Deputy Assistant Secretary of Commerce for Fisheries and Maritime Navigation?
