Exynos is capable of playing back 1080p videos without any conversion, good luck doing that on an obsolete SoC like a sigle core snapdragon.
I was just thinking about how much I miss watching 1920x1080 resolution video on a 800x480 screen.....hmm....
It becomes more important when you want to play more games, which are becoming more and more common, on your phone. Only allowing one set of hardware for WP7 would be foolish of MS. Even setting the Snapdragon as the minimum has its draw backs, WP7 devices won't enter the lower end market until they are years old.
The spec for this year adds the 2nd gen snapdragon, which has a considerably more powerful GPU, its a step in the right direction. Setting the minimum at the 1st gen snapdragon is not bad - when they first released this spec (feb 2010), it was very high end, but by now, its not. So now they can put WP7 on low end devices that will still have a solid, smooth WP7 experience.
While I'd like to see some higher end hardware, their strategy does allow them to reach a wide base of customers, with a consistent experience across them.