I guess it would all depend on motive.

I'm just guessing here, but it would seem like the amount of bombardment needed to eliminate mankind would also render the earth uninhabitable in its existing form. Therefore, the alien race's motive would either have to be purely emotional or there would have to be something very unique about the earth that would still be very valuable following the destruction of the ecosystem.

The issue with the latter is that it flies in the face of the mediocrity principle which is the philosophical underpinning that leads to the conclusion that alien life probable to start with. According to the mediocrity principle there shouldn't be anything unique about the earth and therefore there would be nothing special enough here worth killing all life for (as you, Art, stated as well).

That said, if an alien race were motivated to invade the earth, the likelihood is they would still need boots on the ground at some-point, even if only to mop up the minority that survived the broader assaults. Although, maybe something like V would be more likely, feigning friendship while conspiring towards their own ends.