I've considered it myself. I have been sectioning the PCT during my summers away from school. I think a lot of the responses you received deal less with ultralite hiking and more with hiking in general so maybe I can help there. Weather does play a part, I would consider the setup for some desert-rat running but not for the northern mountains.

my thoughts
Pros
less weight: (pants: 6-12 additional oz) vs (bag: 20-30 additional oz)
less clutter/chore: Fiddleback mentioned this as well.
layer: can quickly be pulled on when resting
Less space: smaller pack
Flex: Someone (sorry I forget who) mentioned this as well, you can own one lighter bag and use your jacket and pants to extend the temp range.

Cons
colder: I imagine this to be like wearing gloves vs mittens, the fingers can't share heat in gloves, similarly the legs and arms can't share your core heat stored inside a bag.

My rambling thoughts... I hope they help! I'll be following the thread and I would love to see others thoughts as well since I've been kicking it around for a few years myself. I just can't pull the trigger on a several hundred pair of pants that may or may not work out!

Brad