The user preferences table contains information which at least should be stored for an person, if not any party.
It stores prefer_text_only_p which is useful if you send a mail to a party, language_preference and locale (reason given above), dont_spam_me_p (also useful for anyone with an email, read: parties), email_type (well, it says email), timezone (usually somthing you want for parties as well).
Therefore I ammend my TIP and ask for a change where "user_id" references "parties" and I write a big warning in the SQL file that this it is called user_id and not party_id as we are too lazy to fix it everywhere :).
