Coming back to this question because it is a common one and I have found a couple of places where the TP team has addressed it.
This post from Len:
We’ve tried looking at hotel occupancy in our models. It never comes up as one of the top 20 or so predictors of a good general model for wait times.
Conventions, for one thing, tend to increase resort occupancy without affecting wait times much. For example, Primerica has two 5,000-person WDW conferences coming up from 1/16-1/19 and 1/23-1/26. But except for two days where they’re renting out DHS in the evening, those folks are going to be stuck in meetings all day. For the most part, they’re not going to be standing in line at Space Mountain at noon.
This article linked by a reader in today’s Crowd Calendar article:
At 90% occupancy Disney World can serve just 22,900 of the 65,000 needed rooms—just a little more than a third.
Occupancy at the Disney hotels doesn’t flex up and down much with crowds. Rather, Disney runs its price seasons and its deals to hit a fairly high level of average occupancy year round. Hotels fill up in the highest-crowd times, but they also fill up during the rest of the year too–including times when savvy Disney World visitors (the most likely to occupy a Disney space—especially DVC owners) know are great times to visit—like early December!!
In other words, the crowds don’t come from Disney World hotels. They just don’t flex enough. Rather, they come from off-property folk. So full Disney World hotels don’t necessarily mean high crowds. (They can mean that—it’s just that they don’t necessarily do so.)