The way you’re using the SPS is actually the right way to do when using a single material and parts of a single texture.
Now let’s imagine that we want to manage some particles with, say, a standard material with a texture, some others with a standard material and a bump texture and some more others with a PBR materials.
The support of the MultiMaterial will allow to have, say, 300 particles of each of those three different materials, so 900 particles mixed together, and rendered with minimum possible number of draw calls (so 3 or 4 in total according to each material specification).
Of course, the subtextures will go on working, even, with MultiMaterials, so you’ll be able to use both features together 
[EDIT] just because you asked about it MultiMaterial Solid Particle System