Short answer is yes, you do need the RAWS installed to backup remote servers.
While there are obviously ways of backing up a remote server without having RAWS installed on the remote server, for instance as you said using UNC paths, you won't be able to backup any of the system state information (which you may or may not care about), but more importantly (certainly from the perspective of Symantec anyway) you're then using the software without a valid license, therefore making it an illegal installation, which is obviously never a good thing for a company.
Personally I think the easiest way to think of of RAWS is as two things rather than one. Firstly it is the agent software which allows you to backup the remote server including all of its system state information, and secondly it is a license to allow you to do so. The fact that you don't need the first part for what ever reason doesn't negate the requirement for the second part.
