Let me put it this way:
I am happy to add it as a suggestion to the Marketplace Guidelines, I am very reluctant to add it as a rule to the Marketplace Rules. Suggestions can be followed or not. Rules require enforcement by the moderator team, and yes, we do spend a not insignificant time reminding people about the resurrected "proof of ownership" rule (but that's ok - I know people don't read stuff).
I don't think Stoneyrun's request was a poorly thought out one, nor do I think it was invalid, nor do I think they're threatening to leave the forum if they don't get their way (although they may choose to buy ukuleles from a different platform - but they may anyway, for completely different reasons than having a price on the subject line or not). It's fine to stand for your opinion and support it, and to refute someone else's opinion with solid reasons and/or facts, but let's back down from taking and giving offense on this one, it's really not that big a deal.
It seems that most of the responses here are not in favour of enforcing a rule to put a price in the thread title, but if a seller wishes to put the price in the title that it's not necessarily a bad thing, as long as they're aware enough to change it accordingly when they make changes to the main "price" field with price drops etc.