The "why" varies in reason!

For me, the side sheeting of my Nieuport is about four feet long and a little more high of aluminum. For consistency of color and finish (the rest of the fuselage is fabric covered), I'll paint it with latex. I'll prime it with something other than latex just to put a barrier between the two, but I'm not really concerned with corrosion.

1) The underlying fuselage structure is painted.
2) The sheeting is easily replaced.
3) I'm not building a plane for the ages. The paint would have to corrode through the sheeting and then the tubing underneath to cause a serious problem.

My plane has a flying lifespan tied to my own. I'm almost 50; let's be generous and say I fly into my 80's. I'm betting it will take longer than 30 years for a hangared aircraft to have the latex eat through the aluminum to the point where it needs to be replaced. If it does, I'll just cut a new one from sheet and screw it on.

When I stop flying the plane will either be donated to be hung from a ceiling or scrapped - I do not want it sold to another person.