Because /x7 is just that good. The second attack becomes progressively lower value the less accuracy/mods you have, and losing /X7 loses some of the best accuracy options, particularly Juke, which is just tasty on Rexler in particular - giving you a potent offensive mod which doesn't take an action or cost focus is huge for him, because it means he can take a focus as his action and not use it so he can save it for his ability. If you could get Juke on /Ds I would say different, because Juke gets better the more attacks you throw. But I can't think of a way to do it.
/x7 also makes the Defender more... well, defensive, whereas /D paints a giant target all over it.
The 2 free points are just gravy.
/Ds with Rage might be interesting. But the real issue with /D for me is the lack of pilots with any real synergy with it except Vessery; it's why I hope Ryad has an ace ability for it.
I see your points in that, but still think even a generic with /D and Ion Cannon can be effective, especially if it draws attention to itself. Sometimes you can have different ships of different threat levels. Do they go for the generic Defender with /D? Or go for the aces? The generic firing last in the turn can maybe get the hits through on ships that have used their tokens. It adds control to the list. It's still pretty brutal, even if it doesn't modify both shots.
I think if you focus purely on mathwing efficiency numbers, you miss out on other aspects. You can have a lean list, but if it doesn't have other leverage than just jousting efficiency, it can fall flat sometimes.

