People have been pushing their views on others since they could grunt and make scratches in the sand, so it comes as no surprise to me that this practice makes its way into board gaming. It is no less annoying than being told what god or what politician to worship.
Yet some cannot resist the temptation to evangelize their favorite new game, or anti-evangelize (there is no other word) the same.
Some people just like to argue, and be right, no matter how wrong they are. If you say the sky is blue, they will disagree. But evangelism goes beyond that.
Some people like to make others aware of a game that they consider to be a gem but "below the public radar". This is a good thing, but evangelism goes beyond that as well.
There seems to be a need in some people for others to like the games that they like, as if their opinion is somehow validated by public approval. Anyone who says the slightest disparaging comment, or who gives a rating less than 7, gets a quick rebuttal. Insecure much? There's spirited debate, and there's pointless gainsaying.
On the flip side are the anti-evangelists. They either hate a game, or are opposing hype out of principle. They want you to hate the game simply because they don't like it. They are really no different; the motives and behaviors are the same.
In my opinion, the point of a public forum like BGG is to allow others to get information. Opinion is information. The sum of all the opinions about a game are an incredibly valuable tool for research. State your opinion, back it up with information, debate it to the point that others can understand the issues, then get off the podium.