PRT, or virtual texturing in general, doesn't save any memory bandwidth. The GPU only accesses the memory areas (cache lines) that are sampled in the pixel shader. The extra data that might be (or not be) in memory isn't touched at all.
The main purpose of virtual texturing is to save memory. With it you can keep textures partially in memory (fine grained spatial and LOD locality), streaming only texels that are needed to render the current frame from HDD to memory. It can also be used to save processing cycles, for example by caching results of complex decaling or procedural texturing systems (process only the surface areas that are needed, and reuse the results for several frames).