This touches on "opportunity cost" but underemphasizes the capital loss that comes with a developer purchase.
To see what I mean consider this:
Say you buy a condo for $600,000 in cash and get $2500 rent a month ($30,000) a year. So you get 5% return on your investment per year. Is it a bad investment because you can get 5%-10% by otherwise investing in stocks, bonds, or mutual funds? If that was the only consideration then nobody would ever invest in real estate. The condo may be a way to diversify your investment portfolio and, more importantly, your actual return may end up more than 5% a year (which is like a "dividend" here) because the condo may appreciate over time (which would be the "capital gain" part of your return). The total return is the dividend (annual rent) + capital gain (appreciation).
The problem with developer purchases (weeks or points) is that the actual resale value is 80%-90% less then your actual purchase price. You can spend $30,000 on points that are worth $5000 if you try to sell them two weeks later. So the benefit that you may derive annually from the points (the "dividend") is very hard to justify given that there will always be a huge capital loss.
So you can say that if you spend $30,000 on points you can make otherwise $2500 in the market annually so "why bother" but it misses the capital loss part. Like the condo rent, the points presumably provide a "dividend" annually in excess of maintenance fee costs (maybe savings over hotel, rent above maintenance cost, flight savings etc). But it's a very different calculation if those points you bought for $30,000 are worth $40,000, $30,000 or $3000 if you try to sell them in 10 years. The "opportunity cost" makes the comparison vis-a-vis the annual dividend but the most important part of the statement quoted above is "without touching the principal". If you invest conservatively in stocks or bonds for 10+ years your principal is more then likely to stay intact. When you buy from the developer the "principal" gets decimated once your right to rescind is gone.