The idea is that someday browsers will be resolution-independent. They’ll have to let go of 96ppi and probably pixels altogether. Making this change is preparing for a future that may or may not come to be. (The Retina Display hints at this.)

The point is valid if not immediately useful. Practically speaking, if you can start and stay with pt measurements, you might as well.

(Regarding how we might bring in a resolution-independent future browser, it would have to be some sort of meta tag or doctype, or maybe CSS fallbacks as we do with font faces.)