I think I've seen not one, but two separate MC magazine "best ADV bike shoot outs" where the ST drops out because of a smashed oil pan. I guess the GS is always going to win anyway, but still.
That being the case, I opted for an ACD which is frame-mounted. It's heavy, it reflects noise and heat at me, and oil changes are a complete PITA. I only mount it when I'm going someplace to ride - rallies, vacay - when I'm fully planning on bouncing around off pavement.
Every. single. time. I've removed it upon returning from a trip, I've had to beat it back into shape with a 5 lb sledge hammer. The amount of deep gashes and dents it acquires is especially surprising given how NOT aggressive/ talented of an off-roader I am. You just need to hit one big rock, or drop off of one little shelf, or bottom out unexpectedly over some unforeseen whoop and you oil pan is gone.
I've heard they're relatively cheap and easy to replace, but having to do so at the beginning of an epic trip with vacation time burning is worth the preventive cost of a real skid plate, IMO.
The VAST majority of my riding time is in FL, which is flat and paved. So the vast majority of the time the skidder is not installed, which cuts down on weight, heat, noise, and makes oil changes easier.