My rule of thumb when it comes to rails: Put your skis on them once, and they are ruined.
Detuning the edges makes the skis safer in the park. They dont catch edges, you can sketch out a spin if you under rotate and dont catch on rails. Rails make good park skis.
Why are they ruined? When you place a metal to metal contact, the metal heats to a point, it melts partially the edge and thus removing it. The second harm happends at the landing, they get a shock cooling;causing the metal to weaken (that is why edges split after a year or 2).
Why do you considered ruined on all mountain? Unless you can carve, this is not an issue. Slide turns is a beginner technique that does not require an edge to do; you can do it on Xcountry skiing. When you carve, you are using the sharpness of the metal on the snow/ice to help create an extra centrifugal force to overcome the centripetal force created by speed and mass. Remove the edge and you will overcome the snow friction and slope angle; resulting in your legs to skid from underneath you. Reducing the sharpness of the edge expand the surface area of ski/snow contact. At a point, the area will be too large to cut into ice, or making the ski uncontrollable at higher speeds and gnarlier terrain.
What to do? Well, serious riders have a park ski and a mountain ski to avoid damaging the latter. Also, riding 2 pairs of skis extends the life of a ski by half a year...like owning 2 shoes. Second, if your too broke for skis, detune the underfoot only. That will still ruin the ski, but you can still get some fum out of it.