I just don't get it. I for once use Jirachi as my central pokemon (which is my favorite legendary). Why make Jirachi focus only on the Sp. defense when it could go for both of it defenses? I mean, cosmic power increases both, right.

Evs. 252 HP, 128 Def, 128 Sp. Defense.

I'd gave him a type of attack like this.
-Drain Punch - healing purposes and surprise to heatran and tyranitar
-Iron head - just for stab and flinching possibilities
-Wish - obvious of course
-Cosmic Power - ups both defense by one level

Now I know so far it was meant to have a stall match against the New BIRD tornadus and unexpectedly Draco meteors from the Lati twins. But What are the most dominant attackers in OU / Ubers? Are there more special attackers/physical attackers, or mixed sweepers?

I find Jirachi best as a mixed setter you see. I find him more of an offensive mage (SP atk) or a vanguard (Mixed attacker) thanks to his 100 all base stats. Are jack of all trades pokemon mediocre?