Cauldrath, you would think it would be obvious, but even at the 2k elo I often see teams which skew very heavily one way or the other, and are easily destroyed because all they have is ap burst, for example.
As for how often supports are played as supports, I'm not sure how it is at your ELO but at the 2k elo, which is where everyone probably strives to be, and where the closest semblance to skill is, supports are played as support, and they do very well at it. Feel free to read previous comments about the 2 support meta that mollica kept trying to get to work.
As for AD caster, it is quite different. AD casters are unaffected by the slow aura of frozen heart, randuins, wardens mail's slow, the damage return from thornmail, the slow aspd skills of ezreal and malphite. Whats more, irelia is a completely different champion to pantheon. She excels at going through the frontline and diving a squishy, while pantheon doesn't dive very well and should be more orientated towards making good use of his stun for the teams focus fire, and getting off a good HSS onto multiple people. Urgot is similar, he wants to get his E off on to the focus fire, because of its armour reduction, land Q's for damage, and have his W up so that they are slowed and easier for everyone else to kill