Bizarro Sun Yat-sen said:
In other words, they came to Earth cuz alien chicks dig it?
Could be. Point is we don't know. In something like The Predator, it's made somewhat clear that there's an honor/pride/hunting thing going on... not purely killing. In Signs, we have only the exerpts of various quacks (from bug-eyed army recruiters to tinfoil hat wearing "scientists"), who the viewers and characters can take at face value... but how logical is that to begin with? Accepting the first explanation that comes along as truth?
We can only theorize, but take the final alien for example. I think it's pretty suggestive that there's a revenge/honor thing going on. Though his people are retreating, he stays behind to harass the guy who took his finger. Extend that to the crop circles and hand to hand combat. How useful are crop circles as navigation markers to beings capable of interstellar travel? Not particularly useful. But if a cultural aspect- like say, "I'm marking you, and this is where I'm going to prove myself." (some Indian tribes had a similar policy during war- the kill didn't count unless the target was marked and taken in hand to hand combat) then the circles and the combat makes sense.
I mean, if you really want to subdue and enslave you should still use small arms (that's how atrocities in Africa happen... rounded up with guns, but harmed with machetes), if they really wanted to kill they could have done it from space, so it makes sense to look for motivations other than the ones we've been told or quickly assumed.
It might have been for the alien chicks.