Probably the most egregious of those scenarios came from "V", where the aliens invade for that rarest of substances...water.

I tend to think that any reason for alien invasion would have to be fundamentally irrational, based on the equivalent of philosophical or religious motives. Something like "the aliens must be converted to our beliefs", combined with an equally irrational "Only a land owner is a true citizen" might work as a motivation.

Of course now I want to see an alien invasion that is actually a botched "This will take no effort at all- they will welcome us as liberators" operation.