Celebrim
Legend
And I thought that scene was a heck of a lot of fun, and I wish more games empowered players to try that sort of thing rather than playing conservatively.
Which is exactly why the "Rule of Cool" is such a bad rule. Because ultimately, it's a tautology. Telling that if it is cool, it should work tells me absolutely nothing.
Your position is based on the idea that players want to do cool things but GMs are these stingy SOBs that are trying to stop them from having fun. And maybe there is a table out there that is like that, but all in all functional tables the GMs aesthetic of play is that he gets enjoyment out of the players triumphing over the bad guys in an awesome way. Because for a GM, the fun is over if the players lose (I mean actually lose and not merely have a set back) and the fun is ruined if the players triumph but do it in a way that isn't cool. So the whole time I'm running a game I'm hoping for the players to win with style. I want them to do cool things.
The trouble is of course that everyone has their own subjective idea of what is cool. And even worse, the players ultimately don't care that they win with style as much as they care to win. From the players perspective, what is often cool is just a shortcut to win easily, which is actually the opposite of cool. Cool is achieved when the players win in a way that is both believable and dramatic.
The trouble with Legolas's shield sliding scene is that it adds basically nothing to the scene except that it allows Orlando Bloom to film the scene without tripping over his own feet in the way that Legolas or a D&D hero probably wouldn't. In fact, it's trading something that is easy for such a hero ("running down stairs at full speed") for something that is probably harder ("surfing on a spiky rusty shield down stonework"). Legolas isn't getting a lot of real advantage. The real advantage of the scene is that you can do close up of the actor pretending to fire a bow without risking him face planting and breaking his pretty million dollar insured face. It doesn't feel like something that would be a real tactic by a hero. In D&D specifically it gets you nothing. Going down stairs doesn't cost extra movement generally (as opposed to going up them), and only requires a fairly easy balance check because running down stairs is pretty easy - you've probably done it in real life. Whereas surfing down stairs also only requires a balance check, and it's probably a harder one than you are forgoing - otherwise stairs would be equipped with safety boards so that elderly people could traverse them more easily.
So what a player is really asking me in this situation, "Can I cheese your scene with unnecessary showboating and risks that I don't need to take?" And well, no, that's not cool. You as a player can certainly showboat if you want, but you could also just fire the arrows before or as you run down the stairs and it is basically going to work the same.
Save the stunts for when they actually make sense.