It is however, perfectly possible to move in someone's blind area, the concealment trick assumes you're already in their field of vision, and while the system does not differentiate facing, the Perception and Stealth skills are the abstractions for looking in the right place at the right time, and being in the right places at the right time not to be seen. You are assumed to be looking everywhere at once, and by that same standard, you are thus not looking at everything in your field of vision at once.
Plus Fucking One.
When you sneak up behind someone in real life, that isn't concealment. Concealment means they know to a pretty good degree where you are (within a 5 ft. square), they just can't pinpoint you well enough to aim an attack. Hiding means they don't know you're there at all.
And again, the question must be asked: why is an opposed check more immersion-breaking than the tower shield paradox hackTM
? What possible benefit is there in forcing mundanes to carry around tower shields just so they can do what, in real life, you can just do by being stealthy? If the point of the system is to create a reasonably playable facsimile of real life, than shouldn't you go with the rules that not only help build that verisimilitude but also make life easier for the nonmagical people?