Dress for Success

I really want to see District 9. It looks awesome and has aliens. However, I do have one question when it comes to alien movies...

Why are they always naked?

E.T. was naked.
Signs aliens were naked.
Close Encounters aliens were naked.
Independence Day aliens were naked.
War of the Worlds aliens were naked.

So I was wondering why all the aliens who invade Earth show up in their birfday suits? (See what I did there? I spelled birthday "birfday" so that all the kids would think I'm cool.) Are we so ineffectual as a race that all the aliens in the galaxy think it's unfair to come to our planet with an extra pair of underwear?
Really now, if you decided to attack a rival country would you do so without any pants? It might work for a moment, the invadees might laugh, point, and say, "Put some pants on buddy! We can see your ding-dong!" But then they'd kick you in the ding-dong and the invasion would be over.

Besides, we don't have the ideal climate for naked invading. Maybe in Mexico or California you can invade naked, but up here in Canada, in February you'd definitely want some mittens or a toque or one of those awesome snowsuits we had as kids...you know the ones, you stepped in them and then zipped them up from the crotch to the neck...awesome. And your mittens were attached to each other by some yarn and looped through the arms of your snowsuit. Now there's some excellent invasion gear my friends!