the rain thing is something different. with substances you can create for e.g. a toilet room with unique ceramic tiles (some are broken some are dirty, grunge effect etc) if you now like to increase the dirty level just adjust some parameters or do you dont like the tilesize and want more tiles? no problem just adjust some parameters. you dont like the color or you want rectangle tiles dynamicly created (dont forget all the maps also automaticly created like normal, spec, height, etc) everything is no problem with substances. if you always have to edit the basic texture e.g. with photoshop and have to export the maps with tools like bitmap2material it takes years. with substances you can check out different settings in a finger snip. you dont just can use shaders for everything and you dont just can use textures for everything. you need both. but you can create those dynamic water effects or snow effects with substances also. the fact that you can scale your texture resolution in realtime also makes like easier. shader coding is also not the easiest thing.
for now I have to export low and high resolution for low/high gfx settings. with substances you could import your substance and ask whats the gfx settings? its super awesome fancy ultra high? ok export the 1000k textures.
just think about if you have exported every texture (low, mid, high, ultra) and then you want to change something... its like "*gnarf* I have to export all those texture maps again -.-" with substances you say : "ok whats next?"