I don't know exactly how the memberships are being counted. I think the 60,000 figure represents the number of unique memberships (your family of 4 wold still be counted as 1 member).
DVC has tried to consolidate ownerships where popssible, so that owners at multiple resorts would still be counted as truly one member, but I suspect the 60,000 figure (or whatever it has risen to at this time) does include duplicates and is actually somewhat smaller.
We have 4 different contracts at 2 different resorts and have been receiving 4 mailings of DVC documents (4 VMs, 4 guidebooks, etc.)
As for the discount "power" of the DVC membership, possibly a better way of thinking of the relative strength is to consider the number of rooms occupied at WDW on any given day using DVC points.
At most there less than 1700 rooms at WDW/DVC resorts. This represents less than 7% of the total rooms owned by Disney. Even these rooms are not all occupied by DVC members, since many rooms have been "traded" for II resorts, cruises, Concierge Collection resorts and other WDW resorts. Since I dont have any figures to use for true numbers involved, I'll be generous and use a 90% occupancy for DVC resorts, with 90% of that actually by DVC members and an average room occupancy of 5 per room. This amounts to a total of about 6900 total DVC members (including family and guests) actually staying on points on any given day in DVC resorts. This is the number I would use as a basis for any negotiation for discounts (and I really believe the number to be much lower than that).
It would be interesting to have some actual data to use in computing these issues. I know Disney has the data to use- including purchase habits of those staying in each resort (they have a viable sampling from those who use room ID's to charge purchases for some very exact data).
Good question. I'm sure we'll have some other considerations here too!