I see that the gap uses gap.com, oldnavy.gap.com and bananarepublic.gap.com.  Wouldn't a better approach for SEO to have oldnavy.com, bananarepublic.com and gap.com all separate?  Is there any benefit to using the approach of store1.parentcompany.com, store2.parentcompany.com etc?  What are the pros and cons to each?

We have a site that has way too many urls caused by our crawlable faceted navigation.  We are trying to purge 90% of our urls from the indexes.  We put no index tags on the url combinations that we do no want indexed anymore, but it is taking google way too long to find the no index tags.  Meanwhile we are getting hit with excessive url warnings and have been it by Panda. Would it help speed the process of purging urls if we added the urls to the robots.txt file?  Could this cause any issues for us?  Could it have the opposite effect and block the crawler from finding the urls, but not purge them from the index? The list could be in excess of 100MM urls.