We aren't talking about theory here. From a practicality stand-point, how much do you think players would like having practice on stages they don't like or consider legitimate just because they are worried about being taken there? Hell, how much do you think players would like actually being taken there in a tournament, with or without practice?
I would be much more likely to enter a tournament with Luigi's Mansion banned then legal, as unfair as that stigma may be, it exists; and we have to account for it.
Well, we have two possibilities.
A scrub-like person(or a scrub), who dislikes the stage being used, doesn't want to practice on it, and is less likely to enter.
Someone who isn't like the above, likely argues/states their opinion on the legal stage(s) they don't like, accepts things like 'even though my character does bad here, it's good for the game', practices on the stage (using the theorycraft often found in debate comes in handy here), and may or may not go to the tournament, but is much more likely to do so, espicially due to the availability of bans & practice.
Now, it's obvious that you should be #2. What isn't so obvious is that we've been expecting everyone to be #2 for quite some time, and as such why is it unreasonable we still expect them to be as such?
Proof?
"(Standing) infinites should be banned for X reason" "Don't get grabbed"
"X character is too overcentralizing on X stage" "Ban it against them"
etc.
Understand? I do not see why this rule change should be any different? Just learn to play on X stage, or convince your TO otherwise.
It's not like that rule already exists today, except if a stage is legal you get 1 chance to ban it, and not multiple.