We currently have a Win 2008 R2 Server setup with Tortoise SVNSERVE.EXE running as a service. We are forbidden to run a webserver. A couple of projects want to add repositories to our server and suggested we "upgrade" to CollabNet Subversion. What would we gain by doing that?
FULL DISCLOSURE: I am a veteran Subversion developer and a CollabNet employee.
It's difficult to imagine what immediate technical benefit you'd get from running CollabNet's packaged 'svnserve' utility versus the one that ships with TortoiseSVN. The primary difference will be the additional support offered by CollabNet for the version they provide.
I can't help but wonder, though, if the folks urging you to install "CollabNet Subversion" are, in fact, urging you to consider CollabNet's Subversion Edge product. (At the risk of sounding like an advertisement) Subversion Edge offers a free, open-source, web-administered Subversion server in a single easy-to-install package. As Subversion server packages go, it's a good one, but it's specifically because Subversion Edge provides a clean interface for such actions as "Create a new repository" that I'm supposing your users mean Edge when they say merely "CollabNet Subversion". Unfortunately, it is Apache-based (and distinctly not svnserve-based), so I don't think it would pass your strict "no web servers allowed" policy.
I suggest you get your users to qualify what, precisely, they want, since neither of the options they seem to be suggesting offers additional technical value which is overwhelmingly obvious but also fits within your deployment limitations. (Alternatively, see about getting that "no web servers" ban lifted and give Subversion Edge a spin for yourself.)