I don't see how dumping lists with arbitrary FPs onto UC helps either UC or SURBLs. In fact it's one of the bad things we predicted: that a grey list would become a dumping ground with some FPs and some domains that belong on a blocklist, all sitting there underclassified, unchecked or ignored.
Actually, the point of the UC list isn't that domains are "underclassified, unchecked or ignored". It's simply that like other spam indicators, some domains indicate the message *may* be spam and there's a lot of room in *may* to include a list that doesn't hit *only* on spam.
That type of list needs to be separate and scored lower than a list that hits only spam. But it's still necessary-- at least some of us feel that way.
Your rule that a domain may not ever hit in any ham means that a domain that hits 99.9% of the time on spam and 0.1% in ham may never be listed when, in fact, it's still a good indicator that the message may be spam.
Having a URIBL that hits some ham means that I may, as with other rules, have to whitelist more senders. OTOH, not having one means that users will receive more spam, which is complained about far more here than FPs.
Bret