I don't but the "you have to prepare for everything" argument one bit.
I have never considered cloyster while teambuilding. I've never considered jirachi (offensive) while teambuilding. I've never considered heatran while teambuilding. I've never considered scizor while teambuilding. I've never considered tyrannitar while teambuilding. I've never considered latias while teambuilding.
And this is for one reason: I don't play stall, so I don't play reactively. My teams (as a team should) bases its members based around what it needs to facilitate whatever sweep/strategy I wish to execute, not to check x pokemon. Playing reactively with an offensive/balanced team is only asking to either be powered through by offensive pressure (not to mention never getting anything done).
There are two pokemon that I NEED to prepare for on every team, though: latios and keldeo. I've gotten away with having latios-weak teams, though, because of how debilitating that -2 spatt is, so I can frequently just set up after that, albeit at the loss of a member.
Like latios, if I don't have a keldeo counter (how many, exactly, fit succintly on offense?), I lose a pokemon every time it comes in. UNLIKE latios, though, Keldeo can continue spamming its attack. It doesn't need to be forced out. Alongside this, it is much harder to wear down due to lack of pursuit weakness, sr resistance, etc. To top it all off, IT'S STRONGER THAN LATIOS.
Keldeo has the same utility as latios, the only other poke I feel I MUST consider while teambuilding, without a number of absolutely debilitating consequences.
And no, you don't need celebi/gastro/jellicent on every team to tank rain hpumps. You need them on every team to tank KELDEO. Against politoed? A ton more can survive, and then OUTSPEED and either ko or force out. Plus, all of those 2 hkos keldeo can get in rain politoed doesn't even come close to meaning latis, any grass/water/dragon/etc type are checks/counters. That's not even to mention the lack of other coverage (hi ferrothorn)
The only comparable poke is starmie, and it falls flat as well. It is a ton weaker, can't really afford to carry specs nor scarf, and is rather frail. It has suberb coverage, yes, but unlike keldeo it can still be dealt with by blanket walls (ferro, jirachi depending on set, latis, blobs, etc)
Also keldeo is a mixed attacker with dual 130 base stats fully invested...