But of course, that also assumes the investments you make yield enough ROI to take out to pay for your trip when you're ready to take one.
EDIT: Please note I'm not implying you WON'T get the ROI to do this, only that it's possible you won't. And by being DVC members, you know you've already put the money into making those trips, as long as you can also keep paying the MF.
I take to looking at DVC like shopping at a warehouse club (like Costco) instead of shopping in a supermarket. Every week, I get flyers for all the different supermarkets in the area - ShopRite might have chicken on sale this week, Acme might have a sale on cola and cereal, Wegman's will be having a sale on baby food, etc. If I have the time to go to all those stores, I can save a ton of dough. Whereas the warehouse clubs won't have the rock-bottom prices on everything, but very good prices, consistently, and I won't have to worry about taking all that extra time to shop and rely on big sales. If I had the time to do that kind of shopping, great, I could use the money, but that's a choice you sometimes have to make - if losing 20 bucks means I have an extra few hours with my kids to play, is that "worth it?"
Comparatively speaking, WDW might offer good deals on vacation packages or room-only deals, but they require that I'm able to go when the deal is available (whether or not it's convenient to me), and hopefully it's not a limited offer (or if it IS a limited offer, I get to it within the threshold of the limit). DVC affords me an opportunity to not only travel at affordable rates, but at my convenience, without having to worry about calling in time to get that super good deal. Or booking a room BEFORE a deal is offered and then hoping against hope a deal will be offered. Add to that the discount on annual passes and the option of doing the Table in Wonderland dining discount, and the knowledge that we intend to visit WDW quite a bit in our lifetimes, and I have no regrets becoming a DVC MEMBER.