If Peter was alive, Miles wouldn't be Spider-Man. It's that simple. He'd be some crappy "fake" Spider-Man with a fake Spider-Man name, like Scarlet Spider. It just dilutes the name, dooms the new character to failure, and embarrasses the original character. If you want a Spider-Man comic where Peter is alive, there are tons of books for you to read: Amazing Spider-Man, Avenging Spider-Man, New Avengers, and whatever 616 books he's guest-starring in this month.
I'm not even a huge fan of Miles, but the fact that Marvel seems to be seriously trying to make the Ultimate Universe more than just an "edgy" re-telling of 616 is the only reason I have any interest in the Ultimate line right now.