Two things:
1) "Purity of competition" is relevant in other fighting games, but in Smash the community already selects the ruleset, stages, and everything else regarding the competition. Saying that Duck Hunt isn't the best because it promotes timeouts is very much a valid point. And that's not saying timeouts aren't a valid strategy - they are. But they shouldn't be primarily stage-based interactions. Skill-based timeouts are impressive - they actually require the player to do the right thing at all times. But if you appreciate seeing someone win due to factors that don't require them to do much, how can you claim to appreciate "purity of competition"?
Part of the thing in most fighting games is this - when you run away from your opponent, you trade good positioning for safety. If you trade nothing, that's hardly a fair playing ground.
2) On the opposite end of the spectrum, watching tournaments tells me that Kirby's biggest weakness - being camped out - isn't much of a weakness at all. We keep talking about how powerful timeouts are, but very, very few people do that (even at the top level). No one really likes to. Our best Sheik players rush in often - same with many of our Clouds and even Sonic players. They have the ability to force a timeout, but they aren't going to. This is more of a problem when facing the mid-high level players, IMO. But past that, no one seems to care much for running away.