I think the obvious answer is money, but not in the way you might be thinking. DVD's are meant to last forever... therefore, one perfect release, and theoretically, you would never have to buy a title again. By first releasing bare-boned, Pan/Scan DVD's, they are ensuring that any real fan of the movie will buy again. Now a certain company can be evil, and rerelease and rerelease a certain title, each time making it better, or they can later just release a really good, meaty SE release, that is sure to make the fans upgrade...
WhenI first heard about DVD's, before they came out, I heard that they would each allow the viewer choice of Full or Wide Screen, that each would have like 5 different languages, and that they would be divided into chapters, like a CD.
Well, some of those things may aply to some DVD's, but certainly not all, as originally promised. A lot of companies may now be excusing their horrid forst releases by saying, we had no idea that DVD's would be such a success, or that there would be such a demand for special features and such, but one of the basic things they could/should have atleast delivered on, was the choice between full and widescreen.
So, inconclusion, don't buy a barbones pan/scan DVD if it seems like a title that could be rereleased again, because more than likely it will eventually be released again, with atleast a widescreen presentation...
I have bought my share of Pan and Scan DVD's, like Honey I Shrunk the Kids, and wish I didn't