When the spammers write comments that are better than people's comments, why would you want to filter out the spammers' comments?
Because it's spam!
In fact, the standard that any spam filter should rely on is: Is this spam or not? NOT is this good or not?
So even if the comment is really good and useful, then at least remove the link and keep the comment itself!
It could be a serious mistake if a spam-filter relies on "how much good this comment is", and this is another problem.