I had written a long post to counter your arguement, problem2, but basically it boils down to this: No, you don't need a specific pokemon to counter celebi and cresselia. Yes, your team is going to need a way to deal with them, like every other major threat in the OU environment.
Hear hear. Those specific bans are quite illogical... if they're banning hax items like brightpowder they might as well ban the abilities sand veil and snow cloak; it works just like it.
Similarly, why ban specific pokemon that have no fault against them other than being one-time only pokemon? Competitive pokemon battling is not only finding a team strategy that wins, but also finding a way to counter whatever is going to be done by your opponent...
Regardless if it's sleep-talk chargebeaming Cresselia or a CMing Cresselia with moonlight, there's always some way to take it down; outside of overpowering Cresselia, there's also Roar/Whirlwind & Haze to counter stat-ups, and you can yourself stat up. Overall, Cresselia can WALL things, but it can't TANK - it doesn't hit very hard. I recall one fierce battle on Shoddy Battle... I was faced with a sleep talk charge/ice beam Cressy, and it could deal a good ammount of damage to the remainder of my team. After some switching went on, I realized that it didn't hit me very hard at all... Petaya Berry Milotic's Surf was JUST enough to get a 3HKO on it, just what was needed to overcome its rest...
DON'T FORGET with the direction competitive battling has been taken in, a fast-paced offensive metagame doesn't just mean worrying about knocking out pokemon easily, it also means worrying about YOUR pokemon being knocked out easily; resistances + a little defenses go a long way! So what if Celebi can't be hit hard? Switch in a grass and/or psychic resist 'mon and pursuit/attack its *** while it does little to you.
Here's a list I've compiled of the generally-considered OU Legendaries, and common ways to counter them for example:
Azelf hits hard, but is frail and easily dies to Pursuit from the likes of Heracross or Weavile, or can be crippled by the likes of special walls like Cresselia or Blissey with Thunder Wave.
Celebi now I can see specific banning of, just because of it (and Jirachi's) controversial OU vs. Ubers spot. As I said, grass/psychic resists help to stop 45% of Celebi's moves, and 90% of its attacking moves. Pursuit works wonders for Celebi.
Cresselia Once again, Psychic type; a Pursuit from Tyranitar or Weavile nearly KO's the thing if it switches out, and other Pursuiting pokemon damage it heavily as well. Thunder Wave? Heracross definitely wouldn't mind the speed drop for Guts activation. Calm Mind? Roar/Whirlwind gets rid of that easily.
Heatran is fire/steel, both of which are weak to ground. Mud Slap would probably
OHKO it. Water pokemon bypass the steel type and hit fire for SE, as well as resisting most of Heatran's moves. Really, Heatran is a VERY cool pokemon, but to quote the Smogon analysis: "Everything else is threatened by accurate prediction." Play well enough, and it shouldn't be much trouble.
Jirachi, considered for Ubers classifcation for at least a time, is also hard to take down. While difficult to get a good one in-game (Infamous is Jibaku's Jirachi as being very competitive-worthy) they're talking about online battle, aren't they? So it'd be perfectly OK to use Jirachi, if its inclusion would indeed make for a good team. As for bringing it down; Jirachi has Serene Grace and not the annoying Levitate, making any powerful pokemon with Earthquake a threat to it; Reflect only makes OHKO into a 2HKO - enough to let Jirachi do something, but nothing to lose your head over. Physical sweepers down? Find your special attacker and fire away with fire attacks (pun intended.) Jirachi is durable and can escape with U-Turn, meaning it'll usually last - but once the enemy's team is down in numbers, Jirachi has less to U-Turn to, and becomes that much easier to bring down. (Statuses always help too!)
Suicune Suicune was a god in Advance... but with the physical/special split, what before wouldn't bother it now gives it a reason to worry. Seriously, I used this dog on a team once, and it always went down fairly easy.
Zapdos If you have high Special Attack or are named Jolteon, Lanturn, or Electivire you won't mind switching into Zapdos. It has HP syndrome - it would love to have both HP Ice and HP Grass, but it can't, so once you find out which it has you can switch with impunity. (I fought a guy IRL at MeleeFC Diamond where it came down to his Zapdos vs. my Swampert, Breloom, and Garchomp; he had both an HP Ice Zapdos and an HP Grass Zapdos in his PC - had he instead chosen the HP Grass Dos I would've been able to win with Breloom and Garchomp, while otherwise I lost. Yeah, I didn't have Earthquake on Swampert to hit Zappy for SE after it roosted - I was trying a different MS.)
That's about it for OU Legendaries... the others are BL or lower, and have either bad typing or bad movepools that limits them from being OU; that being said, they're BL for a reason - there are so many OU pokemon who can overpower them that they needn't be worried about.
Seriously, if these legendaries are banned from team-making, we might as well ban Tyranitar and Blissey, two equally cheap pokemon - Tyranitar is powerful and sandstorm wreaks havoc with so many teams, and Blissey is just a plain pain in the ***. There's debate about making them Ubers tier for a reason.
EDIT: My post can basically be summed up as this: THREAT LIST. I've seen this occasionally in RMT threads I've peaked in at the Smogon forums, and I must say, they're very nice. While not every thread has one, making one is an excellent way to discover weaknesses from certain pokemon that need to be fixed or fined tuned (Such as switching out someone for Gliscor on a Heracross-weak team)
A great example would be this RMT:
http://www.smogon.com/forums/showthread.php?t=33023 where he posted a list of troublesome pokemon that
could cause trouble for his team,
IF his pokemon stated are taken out; such as no Heatran Team vs. Reflect Cressy = Trouble.
With some calculations a threat list would easily help any team to overcome the so-called "broken" group of pokemon so generously called Legendary, if not at the very least revealing the need to switch out a pokemon for one that could deal with them.
Seriously, if you don't consider legendaries banned for making teams, then there's a strong chance that your team won't be able to handle legendaries if you end up running into one in a friendly match or at a tournament that doesn't play with the "legendary clause."
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