I own a MB Alpine Light parka. I bought it for the first part of an AT thru in 2010 since I was starting on the early side (late Feb), and it worked great there, especially to augment my 20F sleeping bag when the temps got into the teens a few nights. Once I hit Virginia, it was definitely overkill. And as others have said, even on the coldest mornings --- no way would I walk on trail with it on.
For thru-hiking the PCT, I just used a thermawrap jacket, and that was plenty (again, just for in-camp or eating lunch, not to hike in). I added a sized-up thermawrap vest to layer just for the Sierras, but scarcely needed it, despite having a more normal snow year (than this year was). So if you plan a fairly typical PCT NOBO, I think that the Alpine Light parka is way overkill.
For actually walking, just a windshirt was pretty much all I ever needed. At most perhaps layer a vest with it (something that's not down and with a lot less clo than the Alpine Light) or a light synthetic hoody or the like.
Of course YMMV on a lot of factors, I don't mean to suggest that my personal approach is some sort of universal truth (!), but still. Just get into your sleeping bag when it's cold in camp. In the Sierras, climb one pass a day and sleep at relatively lower elevations.
Last year and for all my hiking this year I've used a MB Ex Light down jacket, and I love that --- quite light for the warmth. In areas where I might want a little warmth during the day I also carry my MB thermawrap vest. I'm afraid I haven't used the Alpine Light parka much. Hopefully it will find a use in the next few months for winter camping trips.