@gnoams: Maybe you want the crapshoot that was the end of Living Greyhawk, where, if you weren't a crafting Wizard, with multiple deaths under your metaphorical belt, you were underpowered, no matter how well you were built, to handle the so-called challenges? Gotta love magic item crafting coupled with overcap gold, so the death doesn't count, other than losing a level...
One of the goals of PFS is an attempt to make sure that everyone has approximately the same experience when playing a scenario.
Consider the difference in experience levels for normal GMs, as based on the classes they prefer to play normally. Would the GM whose primary PCs are all complex Wizard builds do a better job running a free-form Krune, or the GM whose primary play experience is with simple melee builds?
Also, if the GM preps the scenario, there is, almost invariably, a section, right up front, with the adventure background, which also usually covers the BBEG's motives and viewpoint, so you get a lot right there to handle things when the scenario, as they invariably do, goes off the rails.
@David Bowles: Some NPCs are, indeed, easy. Some, on the other hand, are not. Several scenarios have "death count" threads, testifying, usually, to an NPC's effectiveness in combat. However, even for those NPCs, some PC builds can be devastatingly effective in shutting them down.
NPCs have the problem of, usually, being armed humanoids standing on their two feet on the ground. That opens them up for an effective martial close range battlefield controller to, essentially, shut them down. We won't even go into what a well-built caster can do to them.
Often enough, even something as simple as going second in initiative can be fatal to an NPC's threat level.
Ledford's true bane isn't a gazillion hit point Barbarian build, but a PC with a good attack mod using a reach weapon with Combat Reflexes. "Never charge someone or something with a larger threatened area than you have." Sure, if the Barbarian gets there, it can hurt. But it hurts more, for the Barbarian, if he gets tripped on the way there. Especially if that happens after they have charged more than one move action's distance.