As an aside, have you personally witnessed better quality using 2-pass encoding? At ~2mbps, I've never really noticed anything in terms of quality... but in the 6-8 neighborhood, I can def tell a difference.
Of course! If it's just a "talking head" then two-pass is useless. But for anything that changes between action and static content (like movies), two-pass is necessary to get useful VBR: extra data overhead from the low-action can be passed on to the action sequences while maintaining the same average data rate.
Generally, the first pass is to determine which sections are static vs. which are busy. The second pass then takes advantage and can adjust the rate. So for long talking head scenes, it may drop the data rate to 1.2 Mbps, and action scenes can be ramped up to 3Mbps+ so the action doesn't look blocky.
