Evaluating the source is most useful when you don't have a good basis to determine the validity of the information yourself. This isn't as crucial as it was pre-internet, since it's not hard for most (?) people to do a little googling to find out more, but there are time where I don't care enough or don't have the bandwidth to find the background. In those cases, if I know the background of the source, I can sometimes easily decide if it's worth remembering or discounting out of hand.
I totally agree with AC posting, but as has been stated, most inflammatory AC comments will usually just get dismissed on the spot, which is perfectly OK since most are deserving of being ignored. The ones that are worthy of being read will stand on their own, and it's not hard for most (sane) people to know the difference.
More importantly, if people tend to look at completely baseless AC posts (or other drivel) and jump on them like they were fact...well, now I can generally dismiss THOSE people as well. Helps me figure out which people have no sense of logic and can't be trusted to come to rational decisions or opinions.