I agree fully that Arxiv is the principle publishing platform for mathematics. However, I have almost always gotten good value out of journal peer-review. It might just be that my sub-field is tightly knit, but I always receive thorough reviews with mostly good questions and suggestions for improvement. And I try to give the same when I do reviews of my own.

Even critical remarks are in the spirit of "You should do this better!", never the kind of gate-keeping bullshit I have seen in other fields.