Search Engine Journal recently wrote an article quoting pieces of a debate that's sprung up about digg (and more generically, the social bookmarking sites) being included in google's index.
Here are the arguments:
Allen Stern:

My belief is that this is not enough to warrant a listing in Google. Since at its most basic sense Digg only offers a link to the actual story, then that story should occupy that position within Google, not the Digg link. I believe content publishers actually lose the chance to see that visitor because the person has to click twice and even understand that they must do that. And I am talking about mainstream non-diggers now, not the group who already understands what Digg is for.

William Burn:
Quote: is acting as a gateway to the great content which would (usually) have been lost in Google’s monolithic index, never to be found or stumbled upon by searchers. Additionally, as a counter to his “the person has to click twice and even understand that they must do that” statement, the interface is very easy to use, it’s almost an exact clone of a Google result (a big blue link/title with description underneath) so the user will already be familiar with it, as a result the user would not be confused by it and will find their way to the content they were looking for; that’s if the content still appeals to them after reading the accompanying description.

Personally, I am a bit conflicted on this issue. Digg obviously has the ability to provide extra search engine exposure for content. If said content is on a site that doesn't rank well in the SE's then the Digg listing can help guide traffic to the content.

However, as Allen said, Digg is just listing a sentence or two of the content. That's not nearly enough and really that listing shouldn't come before the actual content that Digg is pointing to. So, IMO google and the other SE's should figure out a way to rank the original content before Digg's listing, perhaps with the Digg listing (and the subsequent discussion on the topic) as a sub-listing. This could be done for any and all of the major social bookmarking sites that link to that specific story.

What do you think?

I disagree completely. If social networking sites rank well for a particular phrase it is frequently because they are more relevant than other pages for the content. Even if it is just a link to the content then the user has found what they are looking for. (Although the content would ideally rank higher then the social networking site page does).

Also digg has not its own content, a short description and number of comments does not make digg as good source for content.

My thought is, it can be indexed, but they should not use it in SERPs!

FYI: Daily I am getting 2 to 3 book-marking websites link through the "Google Alerts" :(

My thought is, it can be indexed, but they should not use it in SERPs!

Then why bother indexing it?

That's what I was about to ask. Google is a search engine in which pages are indexed so that information can be gathered about the page contents to be delivered in the SERPS when someone uses the search engine.