When Tumblr first came out I started an account and posted little things on the side but didn’t link to it anywhere. After a month or two I realized I had posted loads of content there and nothing to my primary site. So it seemed logical to switch.
Garrett Murray, interviewed by Chris Bowler. This is why I switched completely to Tumblr, too, despite having run my own homebrew CMS for years prior.
I too switched from running my own WordPress install to hosting on Tumblr when I realized that dealing with the constant security updates (especially before WP had built-in upgrade functionality) and spam/comment management was more trouble than it was worth. My old blog is still up, although it’s approaching a year without updates. Eventually I’ll get around to running Clint’s script to import my old posts into Tumblr.