Why no flightless bats?

Darren Naish's never-boring Tetrapod Zoology blog asks why, if brids have evolved flightlessness at several different points in space and time, why have bats, a lineage believed to be 50 million years old, never evolved a flightless form?  The answer may be that bats, which are preyed upon by nocturnal raptors, have never found an environment where they were free to go flightless,  I think his reasoning here is that poorly-flying intermediate forms would be at a huge disadvantage, unsafe on the ground or in the air. Maybe, Naish speculates, birds have to vanish before bats can take the emu's niche.