Yes, solely relying on security through obscurity in insane. Adding it as one technique to your security defences is a worthwhile activity. Especially when, as in the case of WP, it is so easy to implement (for which I thank the developers). I don’t really see how this can be argued?
Security is all about applying layers, this is simply one such layer. Yes you can get round it, as you can absolutely *any* security layer given enough time and effort. That is the point; the crackers/spammers pick the low level fruit because it is easier. Simple as that
In case you are wondering, I’m saying this as a server admin of 7+ years. Whenever I deal with defacement or crack on a software package I can almost guarantee that they have used known footprints when installing it. You can take this to extremes, like any security measure, of course. The time you can spend securing something is almost infinite but you try to hit the big targets first in the time you have. One of these, for me, is removing the wp- prefix.