Rails are usually built out of a high gauge steel, ski edges are not, really the only thing skis do to them is scrape the paint up a bit and maybe ding the steel a bit, this may cause some rusting at the most, but during the summer they get a quick grind and some new paint and they're good as new. So yeah, they're only really "replaced" if they get hit by a snowcat or dropped awkwardly, and even then they're just recycled to new rails most of the time anyways.