I think 50% for a character and 30% for a tactic are pretty small percents to use.Try 70% or so. I'm fully on your side about having a numerical standard for ban-related things, but the numbers just have to be higher.
A game needs to have enough viable characters to be interesting, yes. However, ALL of its characters needn't be viable. SF3: Third Strike has maybe 6 tourney-usable characters, but SF3 is still played competitively to this day. Also Melee, realistically, has less than half the cast usable, and it's still an okay game. But when we talk about allowing a tech/character to weed 6 characters out of a 35-character game, things get messy. Brawl gets called all kinds of names, flame wars begin, and nobody is happy until the process is over, past, and long forgotten.
I think a lot of this ban-wagon stuff, while the result of justified feelings by dedicated X-person mainers, is an unjustified over-reaction to the natural evolution of a tournament game. How many fighting games do you know where every character is tourney playable? How about 70% of the characters? 50%? 30%? I honestly don't know ALL of these numbers, but GGXX:AC is the ONLY game where I know 50% or more of the characters can win tournaments if played properly. MvC series, SF Alpha, SSF2T, KoF '98, KoF XI, SSBM... I'm pretty sure at least 25-30% of the characters in each of those games are unusable (or at least VERY un-wise) in a tournament setting. Somebody more knowledgeable than me, please provide statistics. I am too tired to check the exact numbers right now. Thanks in advance.
In SSBM, Sheik, Falco, and Marth ruin the entire bottom half of the cast (taken together or individually). Sheik's d-throw chain, almost by itself, renders a lot of characters weak and useless at her feet. SSBM lives on, though. Oh, and Sheik has long since been toppled from God Tier.
I think SSBB is just at that critical early stage in a game's competitive development, where some of the fat gets trimmed and feelings get hurt left and right. I'm truly sorry for all the DK mains out there who get new orifices torn when their opponent picks D3. And everyone else facing similar situations. Just bear with it, though, be open to switching mains, and in a year or two, we'll all be laughing. Just like all the SSBM players who called for a ban on Sheik or Marth when the extents of their shut-down match-ups were first discovered.
I am VERY sleepy right now, so my thoughts may not be perfectly coherent. Feel free to comment, correct, or flat out tear my argument apart. Just have fun doing it and consider the basic premise of my argument - it's okay for a few characters to fall by the wayside, as long as the game as a whole still retains a reasonable set of options.
(edit possibly to come in the future)