With -p and -r the images will cache even if they are located on a network drive. The problem is that MR will constantly stream data from the map file as it's required for rendering.
If you have a small subject within screen space that only occupies a few buckets then this will not be an issue. But if the opposite is true and you are rendering on a farm with lots of machines you will notice high bandwidth demand on both the the file server and network.
With 100 nodes rendering full frame subjects with 5GB of pyramidal / tiled textures the network becomes nearly unusable and render times skyrocket. I've seen render times drop from 1+hours to +/-10 minutes after re-locating all the textures locally onto the rendernode.
Votch-
