It's a hard call. The problem is that while in some cases it seems completely ridiculous, in others it really could lead to customer confusion, and that's bad f

It's a hard call. The problem is that while in some cases it seems completely ridiculous, in others it really could lead to customer confusion, and that's bad for both the original company and the consumer. "Just do it", for example, is extremely recognizable as Nike, whose brand is associated (deservedly or not) with high quality. If you put "Just do it" on a shoe ad, people will assume (reasonably) that it's made by Nike. Without trademark protection, it would be easy for another business to exploit Nike's reputation to pass off shoddy goods.