Hi Jeff!
I'll try to answer your questions:
a) i've never really been knee-deep in site-wide-search and intermedia in particular, but if i'm not mistaken this column has been used for a trigger to know when to stuff new/changed content into intermedia.
b) only named objects will get a row in this table, so that would mean bboard messages, files, news, site-nodes etc. We certainly don't need to store infos about mere internal objects which the user will never see nor know that they even exist.
c) well, just copy this info via a trigger to the objects_description table. I'm not sure btw (maybe you can enlighten me on that one) whether the description column in cr_revisions always hold a description of the revision OR a description about changes in the current revision etc. More so, is it sure that description is mostly set to some meaningful text (e.g. first 1000 chars of a text message) or does it always depend on the lazy user to add some description (which he rarely does, it guess)?
