Does anyone only allow SE's to spider archive?

[vprp]

I was wondering if anyone here only has robots.txt set up so search engine spiders only index pages in the archive? I know the archive is very valuable for search engines because it is very content rich. I know one person who has done this and is happy with the results he has received.

I would like to do this but here are a few pro's and cons I can think of:

Pro's:

1. Search engines spider and index more pages
2. No duplicate content
3. AdSense performs great in the archive

Con's:

1. I would imagine registrations are much higher on the main forum. In fact, the archive doesn't even have a registration link. How many people will choose to view the full version?