They were never designed as a base coat. Primer has corrosion inhibitors added to protect the surface.

As far as the powder coating goes until recently they had no corrosion protection at all, newer 2 stage coatings with a primer base are available now though.

You just have to see one old frame stripped to understand, it can look great on the outside but as soon as you get all the finish off it's a spider web of rust that has creeped under the finish.

This page explains it pretty well: http://en.wikipedia.org/wiki/Primer_%28paint%29