There can be, in a few limited situations, reasons that one would buy from a developer and not resale. Those can include wanting a new property (resales won't be aviailable yet) and from some "chains", such as Marriott, where buying from the developer gives you options that a resale won't.
Having said that, you should never buy from a developer (or resale for that matter), until you've done your homework.
Joining TUG is the best way to do your homework. I have been a member since 1999, and it's been worth every penny and then some.
We own two timeshares, and are extremely happy with both. TUG has helped us purchase the timeshares, learn how to maximize our exchanging (in our case, going with RCI was better than II), figure out where we would want to exchange (only members can read the reviews) and so much more.
Julia