Anyone depending on a Kensington lock to keep their laptop secure while they're gone has more faith in it than I would.
So? Not everyone uses their laptop in a high-crime, smash-and-grab environment. Just because it only offers modest security doesn't mean that it can't be used (hint: your standard door lock is insecure, but that doesn't stop people from having locks on their doors). In lower-crime environments, a kensington-style lock is fine, as it's main purpose is deterring crimes of opportunity. (And, no, for those people asking: a coffee shop is not what I'd call a "lower-crime" environment.)