Actually it’s really simple. If someone is on your main page then they don’t have to go to the permalink page to reblog something. It’s only redundant when you go directly to the permalink or if your tumblr has a reblog link on each post already.
IMHO, Tumblr needs a “reblog/like button” for posts that works just like the share thing, but ONLY in Tumblr. It’d be useful because it would cut down on clicking within a site – you could do it from the site’s front page.