When I used to sell camera gear the biggest differention was that DO allowed the same optical quality in a smaller lens.
That was the marketing spiel, but it wasn't the reality. The 400 f/4 DO is, I understand, the least impressive of the Great Whites. It's not that it's a bad lens; indeed, it's probably pretty far up there in an overall ranking of Canon lenses. It just falls rather short of the other Great Whites, is all. I understand every other "serious" 400mm option from Canon outperforms it, including a 300 f/2.8 w/ 1.4x, the 400 f/5.6, and, perhaps most damning, the 100-400. And, of course the 400 f/2.8 blows it away.
If it had the quality, it'd probably be hugely successful. But most anybody who's going to blow $6,500 on a 400mm Great White isn't going to be overly concerned about weight...you know ahead of time that that comes with the territory, and it's not like the 400 DO is lightweight in absolute terms -- it still comes in at almost 2 Kg, after all.
If somebody came to me asking for advice, I'd recommend the 300 f/2.8 + 1.4 TC over the 400 DO...you'd save $500, get a better lens, and not add all that much to the weight. If weight is really that important, either the 400 f/5.6 or the 100-400 will weigh a fair amount less than the 400 DO, produce better pictures (assuming you can afford the loss of a stop), and cost a hell of a lot less. Or, if weight is the concern and price isn't, get the 300 f/2.8 II + 1.4x. And if it's just awesomeness at 400mm you're interested in regardless of weight or expense, either version of the 400 f/2.8 is your ticket.
Whatever niche the 400 DO fills, it's a very small one, indeed.