As DotNetnuke comes with alot of the base site functionality you will need (users, roles, site layout and content), I don't see any reason not to use it.
For the most part, the development framework for creating modules will save you time.
There are probably some free and commercial modules out there that will fit your needs, with little or no modification.
For the most part (depending on the modules you use), your data could be easily portable to other platforms (or useable by other systems).