Timeshare owners are usually not required to join II. Rather, they only have to join II if they want to trade with them. Once you join, however, you cannot let your membership lapse without paying a substantial ($200+) fee.
As a result, the vast number of DVC members would not join II. Those that are interested in the benefits would join, and probably enjoy the bonus weeks (usually limited to offseason or inventory available within 60 days for $199) or Getaways (excess inventory or "cash" ressies from $199 to $1099). Occasional traders or those not very interested in bonus weeks or Getaways could still rely on smaller indendent exchange companies like San Francisco Exchange Company.
Personally, I really enjoy II bonus weeks and Getaways. I recently extended my II membership for $199 for three years AND got a bonus week. I used a Getaway last January to stay in a 2BR villa at the Royal Sands in Cancun for $825. The Royal Sands is likely to be the #1 rated timeshare on TUG next year. More Royal Sands weeks are available for this December. I've also used Getaways to stay in Orlando. For example, I paid just $275 to stay in a 3BR villa at Marriott Grande Vista in early June. So there are some GREAT deals out there. DVC has decided to better serve those members who infrequently trade than those who are interested in travel outside of DVC. This works well for many current members, but I'm sure it also irritates some and even precludes others from joining...