While I track a lot of DVC things, I've never actually done the math of the "invest and withdraw" strategy to compare, thought I'd give it a try.
For simplicity, only going to look at my first contract and only rack rates.
Before anyone mentions cash rate discounts: I've also tracked published discounted rates for our exact stays and with an AP discount we would have only saved 7.5% compared to rack rate (with guaranteed availability, and we all know discount availability is never guaranteed and often non-existent), as we have often stayed at resorts or dates not included in discounts. Public discounts would have saved even less.
For the investment side, let's say my Dec 2016 purchase of $34,868 (after closing/contract prep, after 2016 dues, but ignoring CC cashback) was invested in an S&P500 index fund. If left untouched, this fund would have grown ~80% as of today. I'd also invest the dues annually on Jan 15 (ahead of the actual schedule I pay them on, as I pay monthly).
For deductions, I'd subtract the rack rate (which I've tracked for every stay) on the first day of the stay (or the first day the market was open during the stay if stays began when the market was closed).
All said and done, on December 27, 2022 my investment would have gone $106 into the red. Consumed in just over 6 years despite ~70% growth from the index fund (as of Dec 27, 2022), if untouched.
My combined rack rates were $57,689 and the combined contributions were $43,342, so it was definitely worth something (around 33%). But now I'd have no more future value in the hypothetical investment fund while my Poly contract definitely has future value (both in 'dividends', the stays, and resale).
I was actually really surprised by this considering the relative strength of the S&P 500 since Dec 2016, I would have guessed I'd drain the hypothetical investment in another 5-10 years, not a month ago.