Accidentally unsound is a problem, but intentionally unsound just means that they balanced soundness against other design criteria such as completeness, complexity, and tooling capability, then decided that they were willing to trade soundness for other things.

For onlookers, here's a lucid explanation of soundness and completeness in type systems: http://eschew.wordpress.com/2009/08/31/sound-and-complete/

The author of that article argues that soundness is more important than completeness, but concedes that there are other ways to view the world too.

The bottom line is that most ad-hoc lint tools are both unsound and incomplete, but they are still useful for finding bugs!