I think there are a few keys to prefer raising with a hand like 66 or 78s to AKo. First, you never really know where you are with AK. It's a very difficult hand to face heavy action on a flop like A82, which would seem to be draw free. With the other hands, you likely know where you stand. Or, at the very least, you have a better idea of where you stand than your opponent.

I think the second part of this is psychological. In deepstack poker, if your opponent knows that you will raise 3 limpers with: KK and AA 100% of the time, suited connectors 35% of the time and small pairs 25% of the time (just giving random percentages) - you're going to get a lot more action on your KK and AA (needless to say, from worse hands) than you would if your opponents knew that it was KK and AA 100% of the time, and AK 75% of the time, and nothing else.

Third, with drawing hands you want to build the pot so you can win a big pot when you hit, while not losing a lot when you don't. With a hand like AK, you'd just as likely get away on the cheap. It seems counterintuitive saying this, and then suggesting a limp with AK, but I think the point is that AK will be played like a drawing hand in this situation - it needs to be hit hard instead of just brushed.

Of course, I'm not thinking clearly, so I may be completely off target here. That's the way I'm working around it though.