Is it perfect? Of course not. Is it another layer of protection, sure it is.
I've been working a long time in the "security industry".
Believe me, it has reasons why I call products like WAFs snake oil...
WAFs aren't perfect, no security product is. They do allow you to implement protection against many types of common attacks against your website. This is useful if your site runs applications that you don't have the ability to fix XSS/Injection/etc... issues on (you don't own the code, you don't have resources to do it, etc...). This is actually pretty important as most websites out there run old and/or closed source and/or 3rd party code and/or don't have internal resources to identify and fix every vulnerability 100% of the time. A well tuned WAF provides a decent layer of protection. They also allow you you solve PCI DSS 1.2 Req #6.6 without doing pen testing/vuln testing after every single code change you release.
The idea is similar to using blacklists in filter functions in XSS or SQL protection mechanisms. In theory they could block all malicious but in practice they're poorly written and poorly configured crap that act as more security theatre than anything else. The proper approach is to use context-sensitive whitelists for all client input, not add on layers of what is essentially protocol grep.
>"The proper approach is to use context-sensitive whitelists for all client input, not add on layers of what is essentially protocol grep."
It's regex for HTTP requests / responses. Literally, that's all it does.
>"WAFs are usually viewed as relatively useless as they waste time on dumb attacks (specifically blacklisting) that harms more than it helps. "
By who? References? As I mentioned above, we use WAFs and they help a lot with stupid attacks, because stupid attacks are what most of the attacks are; automated attack crap running on botnets to put up phishing pages on easy targets.