I've been at this from both angles. I've gone so far as to have a VC at II say you could add
DVC. Even though I knew things wouldn't go through, I let them take all the registration info. Of course they eventually decide they couldn't. There are several problems. DVC has a contract with II. II only allows members from member resorts to join and in that sense, DVC resorts are not members.
What members give up with the current DVC setup is that they can only access certain resorts rather than the entire II resort directory. It also means we can't get bonus weeks (so called two for one exchanges), or access to cheap rentals. We also don't get the wish book or quarterly updates. OTOH, we have a cheaper exchange rate $75 vs $121 ($149 international), the ability to search first, the oportunity to be called when a match occurs then say yes or no and no yearly membership fees.
For the rare exchangers who does not own other II timeshare, this is the best option. For those of us that are also members of II and exchange regularly, it is a real disadvantage.
There is no way around this with II but there are other alternatives. There are a number of independent exchange companies and many are far easier to work with than II for those with top notch exchange options. In the case of San Francisco Exchange, they will usually give two for one that are much more useable than II but last I heard they wouldn't accept studios for Orlando.