I get awesome hosting for free and a fair amount of flexibility. Yes there are limitations and the admin of the site isn't quite as easy, but when you haven't got much admin it's ok.
The most important reason for choosing Tumblr was the Submissions feature. Visitors can submit a post just as I need it to be. It costs them nothing, it costs me nothing - all the hard work is done (mostly - I obviously need to do some editing, but this is minimal.)
But the absolute killer feature of Tumblr is the queue. Without this, there's no way I could manage the site.
Would I like more features from Tumblr? Sure. Will it happen? Probably not. Do I need those features? Not right now.
I handle all submissions myself, the Twitter feed, Facebook Page, Google+ page... all done by me (with the help of the awesome http://bufferapp.com/)
I use IFTTT (http://ifttt.com) to check for new posts on Tumblr and cross-post to Buffer (Twitter) and Facebook - this isn't perfect yet because I'd like it to use Buffer for Facebook, Twitter and Google+, but IFTTT doesn't support that yet.
Technically the $10/month for Buffer is a cost... but I use it personally and on other projects, so this is obviated by the fact that I would be using it anyway.
I use the awesome BuySellAds (http://buysellads.com) to serve ads. Yes they take a cut and this too is technically a "cost"... but it comes straight out of money I wouldn't be making anyway so I don't see it as a cost.
I also run with a few affiliates and these bring in a little extra revenue.