I can see what you are saying, Elegan, but while I accept that reading a wide range of styles is very positive in many ways, I still don't think that that alone improves the average reader's grammar. In my own experience, I would say that it only made a huge different to my own language after I had acquired a sound grammatical framework, and, for me, that didn't come from reading alone.
I am saying this as someone who was a prolific reader as a child, but who was only taught very basic (primary-level) grammar at school, and whose grammar only improved when I went back into education as an adult. D
To put this into perspective, despite my love of reading, I had no idea how to use the apostrophe until I was in my late twenties.
But we are all, different, so I can accept that my own experience isn't necessarily the same as everyone else's.