My apologies.
I've read too many posts on various boards where people have been using the "unlimited fastpass" wording to the point whenever I read the free FPs, I assume that's what they are referring to. I assumed and made a ___ out of myself.
No worries.

Does the 350k estimate on families include all family members or just owner(s)? If it's really 350k total, as I suspect for marketing purposes (sounds like a lot of others are "in" so you should be too), then it stands to reason that on any given day, there are on average 1000 DVC members spread through 4 parks. That means 250/park.
There are about 3000 two bedroom equivalent DVC villas at WDW right now with a max occupancy of at least 9. Even assuming that not every room will be filled to capacity, 15,000 - 20,000 guests is a fair estimate for the total population on average. So the numbers are quite a bit larger than your estimates.
Many WDW attractions have a capacity of 1500 - 2000 riders per hour. So even when you spread DVC members out over the 4 theme parks, our numbers are large enough to have a noteworthy impact on the experience of every other park guest. 4000 - 5000 people potentially getting front-of-the-line access to Toy Story Mania and Soarin would not go unnoticed.
Additionally, it's my understanding that DVD actually reimburses the Parks & Resorts division for every instant FastPass given out. DVD doesn't have the ability--nor necessarily the resources--to distribute as many of those passes as they wish.
But when you start looking at it as a VIP perk--perhaps with as little as 5-10% of members participating--suddenly the numbers aren't quite so frightening. 250-500 passes per park, per day can be absorbed much more easily than 5000 per park, per day.