Why include archive URLs in sitemap?

Just checking my Google Webmaster Central stats and warnings, etc., I notice that there are a number of errors relating to archive posts.

As it turns out, I have blocked spider accessd to /archive/ in my robots.txt file as an anti-duplicate content strategy.

Now, although I feel I should know the answer to this, I'm confused about whether

1. to continue to block indexing of /archive and delete the archive links from the sitemap; or
2. to remove the block from robots.txt.

Are there any advantages to allowing indexing of archived posts? Are there any disadvantages?

I suggest disabling the archive entirely. remove it from the site and the sitemap. it's pointless with vbseo.

Thanks, briansol. I've now disabled those pages in the sitemap.

I also donot find any value within vBulletin archive and disabled it completely.