I have to disagree. Even in organized raiding, the number of wipes I've personally seen on progression, that land just short of 5% or less boss health, is a lot.
And the number of PUGs Ive personally seen disintegrate in less than 5 pulls due to poor DPS is astoundingly high. The number of people kicked for low DPS or bad heals or weak tanking is higher.
I'm not saying PUGs cant get it done. Just that it isn't as frequent as you make it sound.