You see, this is where I disagree with you.
In the situation that you describe, IMO you *still* choose Andrew Luck. Why? Because he was FAR AND AWAY the best and most valuable player in that draft. And you do not pass on him just because you don't "need" him.
And why do you not pass on him? Because as soon as you select him, teams are beating down your door asking you what it would take to get him. Or (in this same case) Cam.
IOW, selecting a guy like Luck in the hypothetical you describe adds value to your team because of what you will MOST ASSUREDLY get in return for one or the other of Luck or Cam.
I don't care if you have a 26 year old Tom Brady or Peyton Manning on your team, if a player like Luck is sitting their when you are drafting, you take him.
But in your scenario, he isn't the BPA. Yes, he is a valuable commodity, but not the best player that is available for the Panthers. And I don't believe you take him with Cam already on the roster...it's a wasted pick for a player that you don't envision getting on the field for you. You don't pick him to trade unless you already have a deal in place, otherwise you could get stuck with a talented guy that won't see the field for you, after passing on someone that could be an impact player for your team, or if you do find a partner, very possibly not getting as much value in return as you gave up.
As far as his value, it's how much value other teams place on him as a player. If they see him as a starter, then he is BPA for them and you can make a trade before the pick. The idea is to find opportunities to improve your team. But talking strictly about BPA, a player can only be that if he improves your team on the field. No matter how talented he is, if he's sitting on your bench, he's not helping your team like someone who will see the field. Now as the draft goes on, those players do start to fill needs...as backups and potential future fill-ins.