The assumption is that you might want it on Bluray.
If you want the film on dvd and never intend to upgrade then its money well spent.
But if you buy the dvd when there is a Bluray option is IS money down the drain because only rare or deleted titles get anything these days .
My collection of dvd's to sell is growing as my Blurays increase but the dvd's are so worthless now I'm going to keep them for about 10 years and hopefully sell them as curiosities like Laserdiscs were
Play.com current sale has stacks of dvd's for