They keep Microsoft pinned down and erode its public image. If Microsoft hadn't been pinned down by antitrust action at the critical moment, Open Source wouldn't be anywhere near where it is now.
Unable to attack Linux while they were claiming it in court as a "viable threat", Microsoft now has quite a job trying to put the genie back in the bottle.
Continuous court battles have eroded Microsoft's public image to the point my clients now ask questions rather than simply discarding what I say about the company as the ravings of a loony, and are more open to considering alternatives.