It seems like "the poor man's copyright" comes up in every songwriting seminar, publication, or discussion group I've ever been involved in. I'm not an attorney, but I have been involved in copyright infringement litigation on several occasions as both a party and a witness. The only way to secure a relatively safe copyright in the United States is to file the $30 fee and a completed copyright form with enclosures to the Registrar of Copyrights. Whether you do it by Form SR, PA or even TX, for lyrics, you are in far better shape than trying to short circuit the system.
Don't get me wrong, I'm a great believer at bucking the system at times, but not to cut my own throat. Attorneys and copyright law professors will give you a lot of legalese why you should register your copyrights, but the bottom line is:
1) A registered copyright can save you thousands of dollars in legal fees for discovery and pretrial proceedings. Where I come from a 4 hour deposition costs about $1500 minimum. For me, that's enough of a reason to spend $30 on a copyright.
2) A registered copyright entitles the federal court to assess certain statutory penalties against the person or party(ies) that infringe upon the copyright of a registered song(s). The penalty can amount to $10,000 per violation. That's a fairly decent deterrent to most people and companies not to intentionally mess with a registered song.
3) If you prevail in a copyright infringement suit and have a registered copyright the statutes provide for award of costs and attorneys fees, meaning usually that the other side pays your costs of suit. A review of copyright decisions also shows that the courts tend to award higher damages to holders of registered copyrights than to the holders of common law copyrights. Judges aren't stupid, registered copyrights make their jobs easier.
To me the decision of whether or not to register a copyright comes down to whether one wants to be a recreational or professional songwriter.