Roswell is basically a tool that help you quickly setup a Lisp development environment. You can think of it as Ruby Version Manager (RVM) for Common Lisp. It can be used to install the latest version of the Lisp implementation of your choice (which is really a great feature if you use Debian stable or Ubuntu LTS), easily switch between various implementations, and even switch between different versions of each implementation. This makes testing your Lisp code across multiple implementations a breeze, especially when you use Roswell with CI tools.
That the majority of Roswell developers are web developers and use it to develop, test and deploy web apps are merely a coincidence. Roswell can be used to develop all kinds of applications.
Regarding the fragmentation issue, on Linux there are basically two major open source implementations that most people use nowadays: SBCL and Clozure Common Lisp (CCL). So it's hardly an issue at all.