The short answer is when you have a single quux. Then void quux is probably more natural than quux >> return ().

Not having read hlint's code, my guess is that hlint is blindly flagging instances of blahblah >> return (). Implementing this single quux condition may only be a tweak away, and I'm inclined to think the maintainer would happily accept the patch.

Nevertheless David has a good point about void not being in the prelude.

Also, too many void's in the code raises alarm bells for me.