I think in terms of opportunity cost - what else could I do with those points and is it a better value? We have a small number of points and we want to use them (currently) to go to WDW. If I use them to cruise, or go to NYC, they won't be available for Disney and we'd have to pay cash for that vacation. If I didn't have a Disney trip planned for those points, than using them for another purpose would be a good value - the best value may be renting them or giving them to friends or family and not using them outside DVC, though.
We currently have a family reunion trip planned which will use three years of points. Because the family trip is important to me, we paid cash for a weekend last year so we didn't have to dip into this years points.
So sometimes you have too few points and look to stretch them with cash reservations, and sometimes you have too many points for your Disney plans and want to spend them - and if you want to spend them on options where you might get a better deal renting them and paying cash - they are still your points, you get to decide.