Oh, and to answer your specific question -- If you buy a voluntary unit on the resale market, you lose two things: the ability to trade within the SVN network and the ability to convert your unit to StarPoints (Starwood hotel program points).
If you love your resort and want to go there EOY, then neither of these perks will ever come into play. However, if you want to easily trade to other Starwood TSs, not being able to trade within SVN may be a significant loss in value.
However, it's becoming less of a perk everyday. Why? Because Starwood made a mistake when they created their system. Instead of tying MFs and/or demand, to the number of StarOptions an owner gets, they assigned generic, static values. In some cases, such as Westin St. John, their SO valuations are way below what demand and MFs would indicate. So St. John owners, who are paying a fortune to own there, rarely exchange anywhere else in the network because it would be a trade down. Ergo, few villas are ever available to trade INTO. So while tons of folks within the network have sufficient SOs to trade into St. John, and WANT to trade into St. John, it's useless currency because there's no inventory!
In other cases, like Cancun and Arizona, the SO valuations are way too high. A 2 bdrm villa in platinum season in Hawaii is assigned the same SOs as a 2 bdrm villa in platinum season in Cancun. So if a Hawaii owner, who pays $2500 in MFs, wants to use his 148,100 SOs to trade into Atlantis, he is competing against owners in Cancun or Arizona, who are paying ~$1400 for their 148,100 SOs. That's a crazy system.
Finally, the entire SVN network was created with the idea that the network would continue to grow into sought after locations such as Aruba and Cabo, which would ultimately spread the demand more evenly among people from the East Coast who want to escape to a warm place in the winter. They've completely scrapped those projects or put them on indefinite hold. We won't see another SVN resort enter into the equation for at least 3 years.
So the SVN network is really limited and there's a glaring disparity in demand, and cost of owning there, among their properties. Exacerbating the issue is the lack of common sense equity in SO assignment. That has created a bottleneck into 2 or 3 properties, leaving many owners frustrated and which is ultimately diluting the SVN benefit for SVO members.
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Not being able to convert to StarPoints is less than an issue because the number of StarPoints they give you remains the same even as your MFs increase. I believe Princeville owners get 86,000 SPs if they want to convert, while Kaanapali owners get 80,000 SPs. However, you can BUY SPs off Starwood's website for 3.5 cents per. So even if you can get a great value out of those SPs (and those who travel to Europe, especially, can get great values per SP), you can get the same price per StarPoint right off the Starwood website (though you can only buy 40k per year per couple.)
The Starwood Vacation Network is in a state of flux right now. The top 2 resorts (Harborside and Westin St. John) are impossible to trade into, where the remaining resorts are fairly easy to trade into.