As cool as this would be, They wouldn't have to make it open source to allow you to make plugins. That can be done just by exporting a Plugins API through JavaScript or even through a included interpreter (builtin or otherwise, Python is already available for Android, so is PHP, etc.)
Also @Malikster: They probably don't want a million versions of AirDroid available, it makes sense for a closed-source application as there are many people out there who will take the code from an open-source application, tweak it ever so slightly and then release it as their own
So while I think it would be cool for AirDroid to go open source, I respect the developers for keeping it closed.