YUI sits in judgement over us all.
You can find multiple examples of Yahoo using the term evil as if it were an agreed-upon technical term with a firmly defined meaning. I believe it's very unwise for programmers to blur the line between opinion and fact, but this goes beyond unwise. This moves into the territory of those crazy religious fanatics who won't let their kids read Harry Potter because it contains images of witchcraft.
Photoshop making boobs bigger? That's magic! Photoshops worship Satan!
It's one thing to say "I disagree with this design decision" and a whole different ball of wax to say "our official corporate policy is that particular language features are evil."
Unfortunately, I can't put any real weight behind a call to civility, at this point. I've spent too much time telling Obie to shut the fuck up and trying to summon an angry mob to lynch Chad Fowler. But declaring something evil implies a claim to moral authority, and I think Yahoo's claim has no real weight to it either. YUI, as a framework, is already kind of unfriendly to newbies; damning us for our heathen ways doesn't do a whole lot to roll out the welcome mat.
Against my better judgement, I'm using YUI for a project right now. If you're wondering about my experience with it, I'm going to say check with some more knowledgable client-side people, but my personal opinion is that as a YUI newbie, you should show YUI the same friendly, open mind that YUI shows you.