Yahtzee is first and foremost funny. After that, the things which he says are good are generally good. The issue is that it's hardly a complete list, and many things which are good, he says are bad. Of course, this is because that's what makes his video enjoyable, him being extremely opinionated.
However, I don't think many people take Yahtzee with anything except a grain of salt. Most everyone seems to understand that Yahtzee is just expressing an opinion from a single, relatively narrow worldview (Narrow insofar as it is one persons, and a picky one at that). I think the reason why it seems people automatically get behind Yahtzee is because Yahtzee has expressed a lot of opinions on a lot of games, and if a person happens to be expressing an opinion that coincides with something Yahtzee once said, they will likely reference him. That and people tend to quote or paraphrase him to make points unrelated to anything Yahtzee has said (How many times have you seen a post that is essentially, "Short answer, no. Long answer, nooooooooooooooooooooo..."). When discussing on this site, its only natural to use references that we are sure others will understand, and Yahtzee is likely the single biggest common thread we all share. If everyone on this site thought Yahtzee was full of it 99% of the time, and referenced him for the remaining 1%, that would still be a lot of Yahtzee bandwagon-jumping.
As time goes on, Ive noticed that every once in a while, Yahtzee comes across like he is trying to make an argument that is well grounded in facts and supporting evidence, applicable to the general public at large, and in reality, it's just his opinion, and a minority one at that. I realize that this is kind of his thing, but I do sort of wish that there was more of a sense of him being a journalist, and less a sense of him being one solitary, opinionated gamer who just so happens to argue his perspective in an incredibly entertaining way.