The RealPlayer app displaying the serial number or a "munged" flag is a feature that is there for SOCIAL reasons -- remember the analogy with ripped-cover paperbacks or notch-jacketed LPs. You may have a circle of friends who views such things with equanimity, but for some the disapproval factor will be there.
LOL. I don't think it's a matter of anyone's circle of friends - the history of payments on the honor system is not a particularly positive one. Ask any author of a reasonably popular shareware program what he thinks the rate of payment is among people who regularly use his program - and then realize that he's fooling himself if he tells you much more than about 10%, and in my experience, 5% is usually closer still to the actual truth.
And then, of course, many people will find their consciences assuaged by simply using a player that doesn't read or recognize such a watermark. And all this is still assuming that your watermark is robust enough to survive dedicated, repeated attempts to find a method of wrecking it, which, as I've noted, I have my doubts about...
Wrecking the watermark is a lot easier than forging it -- only if you could forge it could you get away with piracy with no consequences. If you can merely wreck it, you will be in the position of someone who reads a ripped-cover paperback -- it's clear to others that you've gotten the product without contributing any revenues to the author or publisher and have participated in an illegal scam (even if your part in it was not technically illegal, you should have been aware that the person selling you the ripped-cover book was almost certainly breaking the law).