I've been trying to answer this question myself by researching and searching on google but I haven't been able to come with a final answer. I have some hypothesis I would like share anyway:
- The average website intends to reach the higher audience possible.
- User generated content by comments is an efficient way of increasing the relevance and the valuable information of the average website.
- Indexing comments will make any site attractive to spammers.
- Indexing comments is more convenient for new/low-traffic websites than it is for old/high-traffic websites.
- The web in general is currently choosing not to index comments in the average website because of fear to the SPAMMERS.
- The average author of a content-valuable comment would like his comment to be indexed
- Not indexing comments discourages the creation of content-valuable comments.
So, what do you think finally? Is it positive or negative for the average website to allow search engines to index user comments? Why?