agreed.
Hm, might make a suggestion about that.
Reduce the crew grade according to the amount of missing crew, i.e. if 50% crew are missing, crew grade is -50%
Of course, usually crew is missing due to destroyed machinery. That engine, that just blew up doesn´t need crew to keep looking after it, so crew requirement on a damaged ship would probably be less than on an intact one.
As crew seems to be replaced when a ship is repaird in the dock, the missing crew wouldn´t be much of a problem during or immediately after a fight. The problem starts when parts are repaired with on-board parts.
Still, for simplicitys sake, I´d go for a streight modifier.