I understand that, it's nice feature but it makes you stop working on the game, code plugin, debug, integrate, test, get back to working on the game. It's not seamless that's what is lacking the plugin system is good for major works, custom tools etc.
So far I haven't found anything I cannot do with normal events. I have found no burning desire to be able to add my own code despite being quite capable of coding myself.
Please understand this simple concept.. if Scirra did what you asked then Construct 2 would cease to become what it is and loose its original vision. It will be no different to Gamemaker or Unity.
A unique product comes along like Construct 2 and its central concept is codeless development and then people ask to be able to write code? That is just defeating the object and more importantly, missing the point.