[Orienda]

I've always imagined it that 16 is a perfectly executed attack, and that 16 is a perfectly executed defense. This gives a little less than a 50% chance of success. Defenses can be more elaborate to improve this, and attacks can then be made more complicated to offset improved defenses, but at the core, the essence of the contest doesn't go past the 16/16. (In effect, it's a normal success roll requiring a certain margin of success, with penalties if the defense is higher than 16 that can be offset with higher skills, just like normal

This makes opposed rolls similar to real rolls, where improving past 16 allows you to offset penalties but doesn't improve your chance of success.

It comes down to whether or not you believe that defense is something that should be absolute, or simply make it harder for the caster. Given the temporal resolution of GURPS, I'm entirely fine with archmages winning against competant foes by successive blows and catching their foes slipping up rather than just always plowing through them in one second.

I play a telepath in a supers game, and the rule of 16 makes the narrative of telepathic combat much more interesting–larger deltas between effective will and effective skill still give an edge, but only because the stronger telepath has a 50% chance of success and the weaker one has substantially less. Sure, you loose some differentiation if they're both offensively minded, but at that point it comes down to luck of the draw.