The reason one should use an appropriate subject and include appropriate tags in it is so that you get appropriate responses. The tag , be it [COMMERCIAL] or [JOBS] for job postings or [HUMOUR] for jokes or [OT] for offtopic, lets the reader know without going through the entire mail what to expect. Only interested readers need to go through the entire mail.
These are standard procedures which are followed in most of the mailing lists on the web. It makes lot of sense to follow them.