The ships that can take tail gunner already have the ability to shoot out the back so the tail gunner card isn't adding another crew member. When you think it out the card is actually upgrading a crew member already present in the ship so why does it require the use of the slot? The card should read
"When attacking from your rear-facing auxiliary firing arc, reduce the defenders agility value by one (to a minimum of zero). This upgrade does not require the use of the crew slot."
The way it stands now the picture should have Porkins and some small dude crammed into one seat. That would at least be funny enough to use.