This is really true only if you're ascribing to the perimeter model of security rather than defense-in-depth, though. If your systems are (properly, to my mind) constructed, knowing what your infrastructure looks like doesn't provide significant value. That obscurity becomes a nice-to-have, rather than an essential, aspect of your security.

(When building systems for clients, this is something I stress. "We should be operating as if it is assumed that an attacker has a VPN into your network space and has nmapped all your stuff.")

Counterpoint: Knowledge of the infrastructure can be used in social engineering attacks, e.g. to increase the likelihood of success for password spearphishing.

If people with access to your infrastructure can be spearphished for passwords, I would venture to say that you're probably doing other stuff wrong that needs to be fixed first.