Many people don't consider it because they bought DVC instead of a larger trip to Europe or a new car, new deck, etc. I certainly didn't pull any $ out of other savings or investments for DVC. I used my normal vacation funds - regular, not invested, gonna blow it going somewhere (soon) no matter what, fun money!
I suppose I could run the time value of money calculation on everything I buy and feel really depressed all the time. Yikes! I have a budget and I don't worry about the money I spend that I didn't earmark for saving / investing as long as I am meeting my savings / investing goals.
Any $ you spend on anything non-essential COULD be invested. Do some people really figure out how much money they'd have if they didn't buy a new bedroom set and lived with their old one for 10 more years? Just curious.