Mega Khan is banned (I've never liked the term "banned" in regards to a Pokemon; its just been moved to Ubers is all) because it takes almost no skill to use one in OU. People DO look for counters (I gave one in an earlier post), but there's only a few of them in OU. Ubers is an environment where things are much more even in regards to Mega Kangaskhan; it can't sweep everything in sight in Ubers. It's not like everything is banned anyway.
As for your insistance that evasion moves and abilities shouldn't be banned: no one likes losing because of pure luck, which is what evasion moves are. Not being able to be hit isn't skillful or clever, its just annoying.
Edit: Greninja'd. Cassie basically said what I said, but a bit better. XD
And that just illustrated one of my points, people jsut go "Oh, that move is pure luck, I won't even try to figure out a way around it" when there's, what, 15 - 20 moves that will counter it? How is evasion luck based when all it takes to negate it is a large number of other moves? Heck, increasing your own accuracy helps negate the opponent raising of evasion. It's easy to blame it on luck, but when you can counter their move obviously it's not luck, it's called being prepared. Seriously, when a Pidgeot can negate double team (and yes, as of Gen 6 Keen eye also ignores boosts to evasion) how exactly can you say double team is luck based when there's SO many counters? Even if you didn't have any of the moves to counter, there's almost 30 pokemon that jsut by being in battle negate double team by having the Keen Eye ability. Again I say, when you can't counter a move that is SO easily countered it's really the strategy you're using that's to blame, not the move.
I think what you're having difficulty looking at is the perspective of what it means when a SINGLE pokemon requires specific counters to deal with. When you have to use obscure counters to deal with a single pokemon, it changes the face of the game altogether. Devising counters is something that should be done DURING the match.
A well-balanced team should be able to stand a chance against another team provided the correct decisions are made.
Uber/Over-powered pokemon require you to create specific counters for it individually (and counters that may be useless against most else at that). That is why it is too powerful to be allowed in standard play (for smogon).
Outside of smogon, it creates situations where people must forfeit when they see the uber that they are not prepared for (you can't possibly prepare for every uber). If you create a team that can stand up to Kangakshan (without using an uber yourself) then you don't stand a chance against, say, Mewtwo or Kyogre. And god forbid your opponent's team is all ubers. lol
Inclusion of Ubers makes battles over before they begin, even for balanced teams. You either have the counter or you don't. This is why smogon has that system. It's not about figuring out counters, because creating counters for particular ubers would require knowing their team before the battle begins.
Getting the ubers out of standard play allows well-balanced teams to have good fights against each other. The strategical aspect then occurs IN the battle itself, rather than BEFORE the battle begins (in which case it's incredibly luck-based).
What you literally just said is what I said in my rant, in that "anyone can get a good winning streak if they put arbitrary rules on what pokemon and moves can and can't be used in a battle" but I stand firm by my statement of "if a sentret with double team can wipe out your team, your strategy is at fault, not the move" Obviously no one can be prepared for every single pokemon out there, but you don't get ideas for inventive strategies and team building by dictating what can and can't be used, otherwise you get players that have no idea how to handle extremely easy to deal with situations like double team or sleep. Yeah, ofcourse if your team is nothing but pokemon with subsititute, earthquake, power up punch, or whatever high powered moves with items geared to specifically power up one aspect or one single move ofcourse you're going to have trouble with a gastly who knows hypnosis. If you have a balanced team and are prepared not only for a heavy hitting battle but also some strategice moves and teams, you'll be prepared for a huge variety of what the opponents could throw out at you rather than only using the pre approved smogon (or whatever) teams so you know "Oh, I'm playing this tier, so it will likely be this handful of pokemon so I'll just pick the counters in my team" rather than not having any idea what the opponent could be throwing out and you needing to build a team that can handle a big variety of things rather than a small focus.
Again, it's ridiculous that stupid things are being banned (Double team? TONS of counters! Speed boost Blaziken? If my pokemon is already slower than blaziken when the battle starts, there's absolutely no benefit to Speed Boost whatsoever. It doesn't get extra moves based on how fast it is, just throw out some powerful tank that can withstand a hit or two from it, smack it back AFTER it moves and it's dead. It doesn't matter how fast it is, it still only gets 1 move per turn) and like I said it's turning gamers who can't handle extremely simple moves that have MANY counters. It's not luck based at all, it's all up to your own preparedness.