Beats me. The announcement about them disabling it was in late October (
http://support.tumblr.com/post/131951272032/pardon-our-dust-...), and on Nov. 4 they confirmed they were coming back, eventually (
http://support.tumblr.com/post/132577661107/david-wizardries...). There's been no movement since then as far as I can tell.
I think they want the reblog to be the primary form of engagement, but that's silly for a lot of things, because the entire original post shows up in the reblog by default. For a running community (just one example), that's pretty lousy: you don't want to clutter up your own blog just to say "That's really fast, congratulations! Keep it up!" to a dozen other people.