My Sitka mountain pants have built in kneepads. You don't notice them very much hiking around, and they are really nice for antolope hunting when there seems to be cactus everywhere you need to kneel/crawl.
While I'm definitely not a "Western hunting expert" I can say that the Sikta Mountain Pants that I use and the Core4Element Switchback and Element pants I use all have them and after the first couple of times getting used to the size/possible discomfort that having them along is worth every ounce (1 oz likely) plus more. To not even have to think about where you're kneeling or on what but to just move/react to a situation is awesome. Wouldn't think twice about bringing them along on my hunts.
Actually I checked out those knee pads that Aron uses and they are pretty slick. I wear knee pads everyday at work that are built into my overalls so I'm comfortable with the ones that are built into my sitka mountain pants. However if those arc'teryx knee pads wet about 30 bucks cheaper I'd buy them in a heartbeat. Funny how I can spend 60 bucks on carhart bibs every few months but I feel weird about spending the same on knee pads that i could use to hunt with and use for work that will probably last a heck of a lot longer than the carharts.