I like tanks on my team because, without conversation, I make certain assumptions:
Tank - will eat the alpha; may control aggro, may do significant damage
Scrappers, Stalkers - will kill bosses; may kill lots of other stuff
Controllers, Dominators - will mitigate damage overall through control; may eat alphas, may buff/debuff, may do significant damage
Defenders, Corrupters - will...do stuff that's good for the party; watch and see, alpha eating, good damage, damage mitigation, and more are possible but not obvious from just the AT.
I get that there is more complex content but for pugs. just having someone who can eat the alpha and everyone knows they will eat the alpha is a huge efficiency boost. I am very happy to have a tank on my pug for that reason regardless of anything else they bring.