A buy-in period is the best way to prevent people to buy a month and download all of the magazines from a certain period, and those will certainly fall to torrents. The best way to handle this is to give people who made the 12 or 6 months deal full back access, while demanding at least a two montly subscription before said access is granted.
This is an extremely silly attitude, I'd suggest.
The only way, in this day and age, to ensure that you get custom, is to do your utmost to please your customer, not to be placing needless illogical restrictions on them.
It's perfectly likely that people will sign up specifically in order to access back-catalogue material, having heard about it - If they ever do a good AP, this would be a perfect example. Those people will not be accepting of a two month delay before they're allowed to access that material, nor of additional fees to access it. Instead they'll either seek other material, or simply access it through less ethical routes.
Essentially it's the same problem any site with a "back-catalogue" has, whether it's a porn-site, an informational site, or whatever. The only way to get people to stick around is to continually provide them with significant amounts of new material and preferably some kind of service that's simply not something you can only subscribe to occasionally.
The only possible way for the DDI to succeed is for Wizard to do their damndest to provide the absolute best service possible, and ensure that customer perceptions are that this is what's occurring. If they mess around, and perceptions don't go that way, they will lose custom, and lose it fast. Refusing to provide your back-catalogue to new customers is an absolute prime example of messing around, and there's no possibility that it would fail to seriously damage customer perceptions. It's really that simple.
I mean, it's not like you're much better off with two months than one. If you can't convince people to stay subscribed, you're SUNK already. Slightly reducing the amount of money you're losing whilst significantly irking customers is simply NOT a good trade-off.
What I would suggest is a good idea for Wizards, and what they seem to intend to do, is bundling their services, and not parcelling them out. This is more likely to encourage people into trying services they'd not considered, and more likely to ensure long-term subscription.