@6: "No, that's just a fairy tale that trademark lawyers tell their clients to justify their billings"

As a trademark attorney, I kind of take exception to this statement.

Companies *do* need to be incredibly proactive in defending their trademarks, or there is a real danger of losing them. Not just from genericness (which does occur every so often, e.g., Aspirin, or Escalator), but also because of estoppel or implied consent issues which might prevent you from stopping future infringing uses.

I have been in disputes where the other side has pointed to third party uses of similar marks to show that they should be entitled to coexist as well. If you allow a bunch of people to use your mark (or confusingly similar marks), it can really come back to bite you when you do try and defend against a particularly bad case later. Any third party use (aside from fair use) that you haven't addressed can be used against you by the other side.

Which isn't to say that the BBC is right in this case--I don't think they are. But it's certainly not a myth that you need to enforce your trademark or risk losing it, or the ability to enforce it (which amounts to basically the same thing).