I've recently done this on some client sites - it's far neater than including several sets of js each loading their own support libraries etc. The main reason for doing it was speed (each button makes several requests for js and assets), but it does also obviate serving multiple third-party js files, and protects your users from tracking by Facebook etc. which some of them won't be keen on. It's a shame that FB, Google etc insist on hosted js - a nicer solution would just be a link around one dynamically generated image showing the user the same information, though I guess that could still be used for limited tracking by IP.

After trialling the standard js we decided to forgo the dynamic buttons for speed, security and the privacy of our users and use locally hosted images of the buttons or logos with a simple link to the social site. For the readers it's very similar experience, but a much faster solution.