Is a security solution worthless if it can't stop a dedicated attacker? A lot of WAF rules are blocking probes from off-the-shelf vulnerability scanners.

"It's technically better than nothing," is kind of a bizarre metric.

It's like not allowing the filesystem to use the word "virus" in a file name. Yes, it technically protects against some viruses, but it's really not very difficult to avoid while being a significant problem to a fair number of users with a legitimate use case.

It's not that it's useless. It's that it's stupid.

IMHO the primary value for WAFs is for quickly blocking known vulnerabilities with specific rules to mitigate vulnerabilities while they are being properly patched. Ideally the WAF knows what software is behind it (example WordPress, Java app, ...) and can apply filters that may be relevant.

Anything else is just a fuzzy bug injector that will only stop the simplest scanners and script kiddies if you are lucky.

It's merely security theater.

It reminds me of when airports started scanning people's shoes because an attacker had used a shoe bomb. Yes, that'll stop an attacker trying a shoe bomb again, but it disadvantages every traveller and attackers know to put explosives elsewhere.

Every security solution can only stop a certain fraction of attacks.