The other thing that pops into my head is this: Cannon upgrades tend to default to the Heavy Laser or Ion Cannon where I've seen them, with the sole exception of Ten Numbnuts (in order to make the un-evadable attack).
The defender is the first time we've seen a cannon slot on anything which can actually manouvre reasonably well - with speed and the ability to barrel roll, the Defender seems like it would pair up with the Autoblaster disconcertingly well. Yes, that's another 5 points but it could be very nasty against defence-die-heavy ships.
I concur that a pure defender wing isn't going to do much. A lone, higher PS defender and a pack of TIE fighters, on the other hand, could be nasty. You'll struggle to hit on the defender since there's going to be four or so fighters or 'ceptors getting in the way whilst it pulls un-evadeable strafing runs back and forth.