I might have been a little unclear - I'm not currently developing (hope to soon). Of course it depends on your market - if your market is a whole bunch of corporations that have IE8, then use jquery 1.9.x... if 2% of your clients bring in a substantial amount of money and it's worth your while, then by all means develop for them.
But at some point, it becomes a zero-sum game. I believe this is what is happening with jquery: the amount of effort to support new features on older browsers (yes, that includes IE8) becomes less and less of an advantage. Hence their decision to compromise and support two versions of jquery, and have all or most of the new features implemented in the 2.0 version.