Ive always found the power/pick/support system to be pretty accurate. I feel like it describes the classes roles on a team well, and definitely better than the stock categories.

As for "you don't need more than 2 of any pick/support class." I mean, no team will ever need a demoknight or a trolldier. There's also the whole "anything spy can do sniper does better" meme, so there's probably an argument to be made that no team needs any spies at all.

If we're looking at pure optimization, I guess there's probably some credence to what they're saying. It's a decent rule of thumb. But on the other hand: three scouts on 5cp is kinda good, and I've seen some good triple engineer holds. I also think triple snipers is better than people think, it's just that people who would pick sniper third aren't usually good at sniper. Also there's an achievement related to having 3 medics on your team, so technically a team that wanted that achievement would need 3 meds.