A sane organization will do it if the benefits outweigh the risks. An organization with sufficient risks will chose it. Most won't, because the risks (specifically, hard cash) won't be outweighed by the benefits.

But that's incidental to what was my real point, which is that if you do want a separate network, it has to be separate. Tie your two "separate" networks together with integration and you're returning single-points-of-failure back into the mix. Granted, practicality may dictate a couple of those... integrated authentication comes to mind... but you don't want the two separate networks to be all slickly and smoothly integrated for the most part.