Taking the last shot isn't necessarily tied to be "trying to be the hero."
I've seen Rondo take the last shot in a couple of situations. I wish someone would put together the video evidence for this, but this is what I remember seeing:
1. shot clock winding down, not at the end of quarters: his teammates would dump the ball to him with 2 seconds left and Rondo has no choice but to make a last second 3-pt shot. Hero shot? No.
As for end of quarter plays...
2. 3-4 seconds on the clock at the end of 1st, 2nd or 3rd quarter, the ball inbounded/rebounded at the other end of the court: Rondo is the only one who can sprint/dribble down the court in 2 seconds and still put up a shot. Anyone else's shot would have been a half-court buzzer beater. Sometimes in transition, half of his teammates don't even bother to run up the court with him. Hero shot? No.
3. lots of time left, last play of 1st, 2nd or 3rd quarter: This is probably what the OP is mainly referring to. Rondo walks the ball up court, waits until 7-8 seconds are left on the clock before making his move. Against good defenses that don't allow him to get to the basket, Rondo usually takes 1-2 steps inside and looks to pass. But he finds that everyone is standing around at the 3-pt line - no one is cutting to the basket. He'll take the last second shot, but because he was originally thinking to pass, he'll miss the shot because he only makes shots when he doesn't hesitate. Hero shot? Maybe.
But what are the other options? Baby would get stuffed. Sheed for 3 is about as tiresome as Baby getting stuffed. Pierce is either injured (during which he cannot make a bucket) or out. Ray can't seem to get free from his defender when he is the #1 option. KG is usually sitting on the bench during the end of the 1st and 3rd quarters, and when he's in the game, he's been hesitating - passing up wide open shots and easy buckets inside the paint. The shot clock would expire by the time Perk finishes his nasty routine of bringing the ball down and taking one dribble before dunking, laying the ball in, or being fouled. Our offense just stinks when not in transition, but a good transition game isn't sustainable given that our defense is spotty and that our biggest offensive threats (Pierce, Ray, KG) are in no condition to regularly run up and down the court with Rondo.