What is the decision process and what resources are available for
deciding whether a corporate legal department should hire an
intellectual property manager to manage and commercialize its
intellectual property?

You have pointed out one consideration: extracting rents from your own
IP in the marketplace.  Alternatively, you may want to keep your IP as
a trade secret.  There are at least two other considerations in hiring
an IP attorney.  First, you may need someone to protect you from
infringing on existing patents, copyrights, and trademarks.  Second,
you may want to seek such IP not to extract rents, but as marketing
and signaling that your company has a large portfolio, and thus is a
good investment or provider of goods and services.

I could answer your question if i knew more information. But for about
the same price you are willing to pay for this answer, there is a
really great book that discusses this is detail called The Startup
Company Bible for Entrepreneurs ISBN: 09755776-1-1