Personally I like it precisely because it's the new generation of chainmail -- except unlike the old model of chainmail, where shit fell into my inbox and I couldn't do much to keep it from doing so other than sending snide emails back to the people sending it -- I can to some extent select what kind of mass-circulated content I am interested in.

Also it helps me keep in touch with some friends who don't hang on any other mutual sites. When you IM or email someone there's a certain sense of etiquette that you won't do so frivolously if you don't have a standing norm of regular contact. On the other hand, passive contact through a social media site such as tumblr is both non-awkward and effective at keeping up some level of being in each others' lives.

I did certainly have the thought, however, that being ethical about fanart essentially meant barring the use of Tumblr for what its format is best at doing.