I'll try to keep this brief:
The number of bans
has no correlation with the number of stages legal. You could have 10 stages legal or 200 stages legal and the number of bans for the same utility.
Having a ban allows for three functions:
- It allows players a preference (I just "don't like" a stage)
- It allows players to ban a specific stage due to balance reasons that are unique to a matchup or character
- It grants the winner of the match some influence on the loser's advantage
#3 is basically #1 and #2 from a different perspective.
#1 is irrelevant -- preference is fine, but if you allow two bans for preference then why not three? Why not four? five? Why not just allow each player to say "you can pick one of these two stages". Why not just
replay the stage you both struck to originally? There's no logical answer to that.
#2 is the important one.
If you're little mac, you HAVE to ban Kongo Jungle because you'll get outcamped easily -- barrel or otherwise. Less extreme is something like being Greninja and facing a Pikachu on Battlefield. Great stage for Pika, bad stage for Greninja, on an otherwise perfectly fine stage. So, as Greninja, you ban that stage.
Allowing
more bans means that certain types of stages can be banned while others are "safe". If you're Sheik vs. Fox, you want higher ceilings and a medium size stage overall.
With one ban, Sheik gets to ban either Halberd, Delfino, Castle Siege, or Town & City.
Castle Siege has the lowest ceiling, but only on the first transformation. The others are more beneficial to Sheik, so she doesn't have to ban that one -- it's not as extreme as the others.
Town & City has a lower ceiling than average (~50% compared to 54% average), but it's by like... one jab. So, town & city is really fine. Sheik has to worry about platforms, but as long as the player avoids being on those platforms up top then she's fine.
Delfino has some lower ceilings during transformations, but these are actual issues for Sheik because they are easy to get trapped on due to how the stage transforms. Sheik might want to ban this one.
Halberd has low ceilings ALL the time. While not the lowest, it is a constant threat and smart play wont' change that like it could on Delfino. This is also a big stage for Sheik to ban.
So sheik has a choice -- based on preference and danger, a sheik player will ban halberd or delfino (likely halberd). Fox will then be able to take them to Delfino, T&C, or CSiege.
That's normal.
With more than one ban, things start to get skewed.
Now Sheik can ban Halberd AND Delfino. Fox has to choose between Town & City, which is essentially a Sheik counterpick as it's only one jab away from Smashville, or Castle Siege, which has the HIGHEST ceiling in the second transformation!
Fox's counterpick ability is completely removed.
But this
isn't the case when it comes to stages that Sheik likes -- the flat stages with longer base platforms. So Sheik gets an unfair advantage in terms of the actual effect of bans. #1 is the same for both, but #2 is in Sheik's favor.
The only situation that two bans is acceptable is when the function is the same for all parties, at least compared to 1 ban. Having Dreamland (shorter ceiling) and Miiverse (battlefield clone) might make two bans worthwhile for this reason, I'd have to look into it more.