You may definitely purchase any of the dining plans when staying at a
DVC resort, regardless of whether you are paying cash for your stay or got a points reservation from a DVC member or traded in through RCI or II. In fact, unless you've booked cash nights (like through CRO or a
travel agency), you don't even need to purchase a park ticket, as is normally required for the MYW package.
All of the other stipulations still apply - how the plans work, the need to make advance ADRs for TS (table service) if you'll use TS, the requirement to purchase the same meal plan for all room occupants for the entire length of stay, the inability to share meal credits with others who are not on the plan, etc. Some folks say they love it because it's prepaid or it's a great value as it suits their vacation eating style so well.
For us, we'd waste a lot of food or gain a lot of weight or just feel bloated/sluggish or all three. The meal plans provide for 2 or more full meals daily, including soft drinks and daily desserts, but not alcohol, appetizers/side salads or gratuities. This doesn't suit
us. We drink more ice water than anything. We start our mornings with real (!) coffee and a simple breakfast in our condo. Lunches/dinners usually consist of a shared entree, a shared appetizer and maybe a side salad. We never have room for dessert so we share one when we have it at all. We sometimes like to snack all day and skip all "meals." And we definitely prefer to decide where we feel like eating, without ADRs, though we usually avoid busy seasons. Disney's dining plans would cost us more than paying as we go. Obviously, it depends on your vacation eating style.
Look at the menus at AllEarsNet and roughly tally what you'd likely spend, by your vacation eating style, choosing from the restaurants you'll want to try, which are on the list. Include taxes, beverages and desserts if you'd enjoy them but not gratuity or alcohol since they're not included in the dining plan either. Then divide your total by the number of days you've tallied to see your averaged daily costs. That's the only way to really see if it'd be worthwhile
for you. There are several threads on the Disney Restaurants forum regarding the dining plans, with plenty of first hand experience. HTH.