Shield boosts aren't that great when you're up against ships that are somehow able to remove your shields in one shot.
I was interdicted by an AI Asp while flying a T9 last night. Shields went in the first volley. It's not like I've gone for a stupidly low shield either, I have a 6A shield generator which should be more than enough to withstand a few volleys and was prior to 1.2. Heck over the weekend it even stood up to an Anaconda.
Given how quickly my shields were dropped a wingman wouldn't have made much difference either as I would likely still have recieved the same amount of damage before being able to jump away, but it certainly would have felt a little better and I may have actually stuck around long enough to ensure the Asp was killed - which in turn may have at least paid back some of the repair cost (frankly the bounty should be 2x the cost of repairing damage caused but that's another debate for another time).
As for trade dividends, they may not be worth it for a single escort, but it's certainly enough to warrant traders to group up instead. A wing of Type 9's could be gaining an additional 200,000 per round trip. That might not sound much compared to how much the T9 brings in, but it all adds up.
It's not as if 4 Type 9's would be easy targets either; even ignoring forward firing weapons, that's a potential 12 medium beam turrets. Even in the scenario above where I lost my shields in 1 shot, that Asp would be equally as toast - especially considering most AI pirates tend not to have shields.
Agree with everything except your comment regarding the ASP interdiction. You must be doing something wrong buddy, I don't consider AI a risk at all. Been playing since early Beta. If in a T9, immediately switch FA OFF, boost and apply upwards thrust (your boost speed will not bleed off) chaff & Charge FSD and jump. That even works for me against some commanders.. traded in a T9 for months always in OPEN.