Would they be useful? It's not currently possible to quickly search for whether or not a term (e.g. patchid:) is present in a Xapian document. Having the ability to do so would make it easier to find non-patch messages, or easily filter down to cover letters, bot replies, etc... Thus adding Xapian terms is probably necessary for that. replycount would be useful for implementing search for ANSWERED/UNANSWERED in IMAP. However, proper indexing/reindex could be a PITA due to out-of-order message reception. I don't think any of these would be required to get "lei rediff" working on entire patchsets, though (it only does individual messages, currently).
