As your example proves, and many were warning WotC at its onset, that what would likely happen is that players would not each subscribe to DDi, as they only need one account per group, not matter what WotC wants them to do.I would say that in most groups, there is a sort of multiplier effect going on. There may be one or two in the group who maintain account, but the group as a whole uses it. My own group was a notable exception, where all but one of us are professionals who have real jobs, so we each had our own subscriptions.
Now it's been a little over a month since I've dropped my subscription, so I'm in the situation of asking others to look something up in the Compendium when we play (for instance). So far, it hasn't been a problem otherwise, largely because WotC isn't releasing any new content. We'll have to see how that changes when -- or if something new comes out.
I think WotC has a notion that they'll somehow be able to come up with a system where an entire group will want to subscribe to DDI, but for a face to face game, I can't see what that would really be.
Saying the average group is 4 players (DM included), then everyone gets everything for a few dollars a month as opposed to full price.
With new material being based off of Essentials, where does this lead to with the gap, as it might require a new Character Builder, or Essentials just wont work within the existing one very well that will further widen the gap.
DDi is really like the pizza ordered. Everyone can use it and pay for parts of it, but you only need the one.