"Artists Beware"

If my lay understanding of trademark laws is correct, there also needs to be, well, trade being done using that mark, because the point of trademark is to protect against business losses caused by deliberate or accidental infringement of one's business identity. If one isn't doing business, then by definition there can be no loss incurred. And, as you note, in order to protect against that infringement, the trademark holder MUST defend it or risk having a court decide the trademark has been abandoned. Those are two reasons why Disney is so adamant about taking even fan art to the courts; they make far too much money to risk any decision that might allow someone else to make money off Mickey, Donald, etc.