[Brodrik]

I haven't seen about whether or not higher-tier ones give you more points or not. I would, however, encourage you to actively work toward finishing them all, as at some point that's got to be useful. :-)

Some of the higher-tier ones seem substantially less annoying than the lower-tier for the same area.

As far as going down over time, I think it's just that you receive more Assault Points the first time you do the mission.