There is also negative "intangible value" - DVC encourages people to take trips they might skip without the timeshare. Even if you intend to go to Disney every year, there might be a year money is really tight and you skip it - with DVC you may make that trip work. We saw that over and over again in 2009 and 2010 around here.
As Dean said, there is value that equals negative savings - how many of us have treated friends and family to a room? How many of us would do that with cash reservations? How many people have discovered that a two bedroom is so much nicer than the studio they ran the comparisons on? Or started with the number of points they needed to replace their cash trips, and then bought a few more (we can sneak in a weekend over Food and Wine, or in Early December), and now end up going more than once a year.
Too much analysis and you don't act. But when you cherry pick the good part of the analysis, you end up in the situation we've seen here over and over again - a young couple who can afford DVC in the short term and buys it for the family they hope to start soon, but forgets that kids are going to cost money - probably a lot more money than they expect. Or someone with a mortgage with a balloon. Or someone who is already towards the financial edge driving an older car - the DVC payments are fine until the mechanic says "this one isn't worth fixing anymore" and they need a car loan and then the next week the furnace goes out. Its easy to get emotionally invested in the purchase, and then get emotionally invested in the ownership - and hang on to long.
And I've seen a lot of something else as well. People who did afford
Disney vacations every year while their kids were growing up and never felt a financial pinch. Who are getting the family contribution for college and saying "how am I supposed to pay this?" Or hit retirement age without having managed to save quite enough to be as comfortable as they would like.
Now, I'm very financially conservative - and my own "rules" around DVC are pretty different and would result in a lot less people owning. But I've also seen people jump through hoops to justify what they want where you don't need near my level of conservatism to see that its a bad idea. Like, Dean, I've spent a lot of time watching these boards - and out there right now is someone who is cherry picking information...who is looking at the person who took out a loan (that they could afford) and saying 'it worked out ok for them.' We don't tend to get a lot of people who have to sell after a year or two of ownership, having taken a bath on the deal, hanging around telling us how it didn't work. Or someone who owned for twenty years saying "you know, I could have maybe skipped a few vacations so my kids didn't have such burdensome student loans."
The end result - you can either afford to sink around five figures into future vacations, and then commit to $1000 or so in dues, plus vacation costs, every year for years (probably plan to ten), or you can't. If you look at that total and aren't seeing value - don't buy even if you can afford it. But if you do see value and you can afford it, go for it. It isn't the cheapest option, but I'm not driving the cheapest car I could either.
(ETA: We talk a lot around here about how much this costs - and almost nothing about the other end of it - how much each of us has to spend. i.e. we almost never discuss income or net worth. I know that there are some pretty well off members posting on this board who can "afford" pretty much whatever they want. And I know how easy it is to fall into the trap of "well, if they can afford it, I can too." When you have a seven figure net worth, throwing $10k at a timeshare purchase is not something that is going to make or break you if you don't squeeze every dollar of value out of it. When your net worth is -$20k in a house that is under water, it might.)