I own a Marriott and DVC...
With Marriott, we HAVE to belong to II even to trade within their system. So, there's the membership fee in case we want to trade, then additional fees even if we want to go to another Marriott.
Its funny, because DH and I appreciate that we have trading options with DVC without all of the additional fees. If you look through the II offerings, many of the properties aren't up to Disney/Marriott standards. We actually are looking to sell our Marriott property for a number of reasons, not the least of which is we're tired of all of the add-on fees.