The purpose of a competitive ruleset is to test the skill of the player in the game. However, if an element of the game makes testing this either impossible (due to randomness messing with the test results, such as on Warioware in Brawl) or trivial (due to the skill ceiling being phenomenally low, such as on Temple), then clearly the ruleset has failed.
- Temple Hyrule is the prime example of a degenerate state. A player adapting optimally to the stage might as well just not pick anyone other than Sonic or Fox, because the stage has a clear optimal strategy that is virtually skill-free. Here's a good metric - I am not a top player. Would I have a decent chance of beating, say, Mew2King on that stage if he didn't switch characters to abuse the optimal strategy? If the answer to that is anywhere near "yes", then we have a problem.
- Port Town Aero Dive is another great example. A player adapting optimally to the stage will still need to know how to play, but there are many mechanics thereon that make small mistakes incredibly unforgiving. Throwing your opponent into the path of the moving cars is a degenerate strategy. Couple this with the randomizing factor of one segment where you cannot see the cars coming more than a second in advance, and the stage is very bad for testing a player's skill.
- Pokemon Stadium 2 is the prime example of a stage that encourages player adaptation without degenerate game states. A player adapting optimally to the stage still needs to be really, really good at this game. They need all the same skills they need on any other stage, plus they have to understand how to fight in a low-gravity state, plus they need to know how lowered traction affects their character, plus they need to figure out how to maneuver around the rollers. The stage does nothing to take away from the game's depth - instead, what it does is increase it. Another simple rule of thumb - would you feel like you have a better shot against a better player on that stage than on, say, Smashville? If not, there's probably not an issue with the stage. And such is the case with Pokemon Stadium 2. It's a prime example of why "different" does not mean "banworthy".
You really can pare the valid reasons to ban a stage down to "Degerenate gameplay states" and "randomness". This cuts out basically everything that
could interfere with legitimate competition. The problem is, many people seem to think that stages that move interfere with legitimate competition. That stages with hazards necessarily interfere with legitimate competition. That stages with odd layouts necessarily interfere with legitimate competition. That stages which occasionally break up the neutral necessarily interfere with legitimate competition. That stages which change the physics of your character temporarily
are inherently disruptive to competitive play
. This simply is not the case. We cannot immediately disregard
any stage just because it's weird and different, unless we can find some degenerate or broken strategy, or some significant randomizing factor.
TL;DR: If your ruleset doesn't have PS2 legal you might be bad at ruleset design.