Yup, the tank was in Newton. And it wasn't good for shit during that mission. It was more of a pain in the ass to manuever the damn thing around obstacles than to just leave it at the start, IMO. And it could only carry 5 people, so you had to leave 1 person out of it. Actually, 2 people when you have one of the 4 Reavers with you. So far, the only vehicle based mission that was really fun was the APC mission. Partially because all of the people could fit in it, and partially because it was highly effective during that super mutant encounter.
As for the encounters that happen ALL the time outdoors, there is a bug saying that the encounters are somehow related to frame rates. I would guess that this means on faster systems, you have even more encounters. (I have a P3 800 and get them about twice per square on average.) So hopefully that patch comes out soon.... Like once Easter is over, maybe they'll get their asses in gear?