I don't have an opinion on the relative size of DVC, other than it's obvious that continued growth in membership will increase the importance of home resort, especially at the smaller resorts.
However, I'm sure overall expansion of DVC makes sound economic sense to Disney...especially when compared to alternative investments like building new hotels or theme parks. The hospitality industry is very seasonal, and also very sensitive to the business cycle, so investing in some aspects of it are much more risky than others. Anybody can build a cheap hotel a few miles down the road, and when the business cycle turns down, theme park vacations are one of the first things occasional visitors are likely to cut back on. We've all heard the rumors that Disney is done building hotels.
DVC, on the other hand, creates a large pool of people who are likely to visit at least once a year, no matter what. Plus, when we visit, most of us probably stay longer, we're more aware of what's available and therefore more likely to do things like behind-the-scenes tours, Segway, better dining, special events, etc. We've all seen the posts which say DVC saves money on lodging, but overall many owners spend more now than they did before they joined.
As far as regular seasonality, all you have to do is look at two time periods to see the effect of DVC -- Food and Wine, and early December. Both formerly doldrums, now you can't find a room anywhere! That works for Disney.
Also, I think the entry of respected names like Disney, Hilton, Marriott and others, plus the flexibility of points-based systems, is radically changing the timeshare industry. I'm sure there are still a lot of con men still around, but I think many average folks lose their fear of timeshares when they see big, reputable companies coming into an industry. I've had friends buy vacation homes specifically because they were afraid of timeshares, and now a lot of them are starting to reevaluate.
With all that, I think DVC will remain one of Disney's best investment alternatives for some time to come.