The word "mandatory' simply means that membership in SVN, Starwood's internal exchange network, is mandatory and cannot be separated from ownership. If you own there, whether your buy from the developer or on the resale market, it is mandatory that you join SVN. All SVN memberships include a complimentary II membership.
"Voluntary" is kind of a misnomer, because unless you buy your unit directly from the developer, you are not allowed to join SVN. We should have called it a "Conditional" resort, because SVN membership is conditional upon purchasing it from Starwood. Because SVN membership is not included with resale purchases, II membership is not included either, though you are certainly free to join II on your own.
Now if you want to get into Westin St. John at all or into Harborside or the Westin Hawaii resorts during the summer/holiday weeks, you should look at either buying there and/or buying a mandatory resort on the resale market so that you can trade into them via SVN. Because of the high demand for these handful of resorts, owning there or trading in via SVN at exactly 8 months out are the only ways you're realistically going to get in these resorts during peak seasons.
Summer/holiday weeks in Hawaii and WSJ aside, if you want to exchange into any of the Starwood resorts and you do not belong to SVN, you can use II. As you may or may not know, clout is everything in II. If you deposit a strong trader, you will "see" more units from which to choose from. Now if you happen to own a Starwood unit, you have two things working in your favor: 1) Most Starwood resorts tend to be strong traders in II, whether they are mandatory or voluntary resorts; 2) ALL Starwood owners get a 2-3 day window during which only they can see other Starwood units.
So, depending on your wants, budget, and travel habits, it might make sense to buy a mandatory resort with their higher purchase prices and MFs OR it might make more sense to buy a voluntary resort with their lower fees and higher clout in II.
Hope this helps!