I think Pierce was looking for Ray, and he was also looking for a backcut, but neither option presented themselves.
The reason you have the ball in Pierce's hands is that he is the best combination of ballhandler, passer, decisionmaker, and scorer. He gives you the most options, depending on what the defense shows him.
Ray is actually a pretty mediocre ballhandler under pressure, and has lost just enough quickness that he struggles creating space off the dribble, and is not a great passer. Rondo you certainly don't want taking the shot when the defense takes away the passing options. KG is great, but he can't make his own shot that well anymore.
I actually think the shot Pierce took was option C or D. He first looked to drive on Duncan, who didn't give him a lane. He looked for passes. He also was expecting for them to give a foul, and it threw him off a little when they didn't.
The tough part about last possessions is that generally defenses are pretty focussed for them. That makes it really hard to do anything complicated without ending up without even getting a shot up. So, if you can put the ball in the hands of a player that you know can get a shot up if nothing else presents itself, but can also search for a better shot first, then it is an ideal situation. It is never going to be a high percentage play, just by the nature of trying to score when the defense is completely set and focussed. If you can just get off a shot which isn't a contested 25 footer, than that is half the battle.
Or let me put it in completely made up numbers.
A shot with a 25% of going in, is still better than a shot that has a 35-40% shot of going in (everything will be contested, you can't count on an open shot), that comes after a pass that has maybe a 15-20% chance of being deflected or intercepted by a defense that is zeroed in.