You sound like you're dismissing it, but even if it wasn't all that useful on its own, it's a part of defense in depth strategy - it's just one layer in a carefully thought out system. Pledge/unveil is another, so is privsep+imsg, W^X, (K)ASLR, syscall origin verification, boot-time libc/kernel relinking, and a couple dozen other features I can't even recall now.

Most importantly, all of these features and mitigations are enabled by default, and are pretty much invisible to the end user or administrator; and actually easy to use for a developer. Contrast this with e.g. seccomp or SELinux. Google is even hinting "selinux permissive" and "selinux disable" in top 3 suggestions...