Silver PLMs have a very focused output, with sharp falloff outside the main pattern, But because of it's size, the main pattern is quite wide in terms of area actually lit. When the white front diffuser is added to PLM it's angle of coverage is vastly increased and is similar to a softbox.
Based on the articles that I've read and Youtube videos I've seen about the PLM, I get the idea that does the job of most other modifiers of its kind much more efficiently. That job is to increase the effective size of the light source. Instead of diffusing the light as it moves toward the subject as soft- and octaboxes do, it simply reflects the light parabolically (is that a word?) so that the rays are parallel to each other moving toward the subject. The intensity throughout the beam is relatively equal, unlike the other modifiers.
Now given that it has greater efficiency, and you get a similar effect to soft/octaboxes when the PLM is used with a diffuser, is there any reason to use anything other than PLMs? Do grids on those other modifiers really change the light pattern all that much?