The purpose of summer waxing is to prevent the plastic bases of the skis from “oxidizing” and drying out.
Technically what happens is the compounds that make your p-tex base a plastic evaporate from an unprotected base. A good illustration of this is the plastic dash of a car. As the dash oxidizes, the plasticizers become volatile, dissipate into the air (often forming a film on the inside of the windshield), leaving behind a surface that looks dull, even powdery.
Ski bases have been developed to be very hard, yet at the same time very porous. These pores are actually a network of open areas in and around very fine strands of plastic. When skis are glide waxed, the hot molten wax drops into the pores of the ski base. By simply changing the type of wax in the pores of a ski base, the ski can give optimum glide in a variety of snow conditions. When the surface layer of the base becomes oxidized, not only does the plastic become less slippery, but it also becomes harder for wax to penetrate into the ski. The net effect is the ski does not glide as well and the application of wax will probably yield little improvement because the wax simply isn’t going into the base.
The ideal world and the real world are seldom in sync. Most skis seldom get a coat of summer wax. They probably also don’t get waxed often enough during the ski season and in general could use at least an annual overhaul. As a general rule, unless skis have been protected during the summer with a layer of wax, they will need to have the bases re-prepared. During ski season, skis that are periodically waxed probably do not need any additional care except for waxing.
Note: This article was originally written for nordic skis. But P-Tex is P-Tex, whether on an alpine or a nordic ski so the same principles apply.