I'm not even sure what you're asking here...
From how you continue your post, I think you are...
Terrain isn't metagame...in combat of any era, terrain factors massively into tactics...your ability to see or not see someone (and conversely have them see / not see you) has a huge impact.
True. But a 4E Ranger is encouraged to move in ways completely contradictory to common sense, having some bizarre supernatural ability to hurt some people but only if he can't see other, closer, people; even if he knows they're there!
It's completely jarring. And from how you phrase yourself, it seems you agree.
Now, on that note, if you're concerned about the technicality that a ranger could have a situation where he has 2 bad guys...one is 3 away the other six but he wants to quarry the further one...so he moves behind a column to give the one 3 away total concealment so that he cannot "see" the nearer one and thus the one six away is the closest he can "see", this raises the question of whether this is concealment or cover...and does awareness count as seeing? And is taking that time to move to gain cover worth it? and do we care if the ranger is able to switch out his quarry like that?
Involving cover and concealment would only make a big headache grow into a monumental pain in the ass.
One easy change is from "that you can see" to "that you're aware of". In much the same way Invisibility have always dispelled itself on the intent of its caster (i.e. it isn't the nature of the action that's important, it's whether the caster himself views it as offensive/hostile or not).
Whether moving around to exploit this "feature" of Quarry is worth it or not is not the issue. Whether the DM can stand having a player who even occasionally would spend time on optimizing such a silly parameter is.
You're right in that the easiest change, of course, is to simply drop this requirement. But I guess the dev's realized this would make it too easy just to stand back shooting down things.
So what do you think of the "that you can see" to "that you're aware of" change? (Or should I open a new thread in Houserules for that?)