You have hit on a $25,000 question. Your answer to this will depend on who answers.
Mush of the debate centers around how you value your points. I usually compare my points for what I paid for them which is aournd $5.50 per year. Others will value their points on what the current market value is. That is somewhere around $10 a year.
When I look at whether an exchange is worth it, I look at the points I have to give up and what it cost me. This makes the cruise aj fair trade.
Others look at what they could rent their points for and what the cash price of a cruise is for. In this instance, most times the cash price wins.
I didn't buy DVC to rent it out. It is just a big hassel to rent enough points out to pay for a cruise. (IMHO) Plus I am paranoid about the possible liabilities of renting on that scale. But what is right for me is not right for others.
Finally I get more than a $10 per point value by staying at a DVC resort. So to me It is a better deal to pay cash for a cruise and bank the points to the next year.