Some member perks are extended solely to the owner of the contract. The most obvious example is the
DVC discount on Annual Passes. That perk is available only to the DVC owner and family members living at the same address.
The main reason to consider buying, IMO, is cost savings. For example, consider a Studio room at Saratoga Springs during Value season:
Cost to reserve direct from Disney: $269 plus tax.
Cost to rent points (weekday): 11pts @ $12 each = $132
*Cost as DVC member (weekday, annual dues only): 11pts @ $3.98 = $43.78
(*That's assuming you've broken-even on the initial investment, which usually takes less than 10 years depending upon how efficiently you use your points.)
We bought into DVC in 2003 and I currently estimate we will break-even on the initial purchase either in 2008. That means it will have taken us about 6 years.
Renting points is still a viable option--particularly if you don't like having the pressure of a forced vacation on a regular basis. Visiting WDW isn't cheap. IMO the vast majority of members should be people who visit at least every-other-year
consistently. If you visit any less frequently, change your travel patterns as finances dictate, or are uncertain whether WDW will still hold appeal in another 10+ years, buying into DVC is probably not a wise move.