My reasons for writing a plugin/behavior are these:
1. Adding a feature that events can't do.
My paster/canvas plugins are an example of that. They add the ability to draw to a texture which isn't possible otherwise. I think another one was a spritesheet plugin.
This kind of blends with one sometimes. Examples include the chipmunk behavior and the peerjs plugin.
3. Speed improvements.
The only example I had for this is my isometric behavior. I had it working with just events, but since it had multiple nested loops it wasn't very fast. Actually I think the sorting algorithm was O(n^3) worst case. There is some overhead with the event system so converting to js allowed me to sort 3x as many objects and still not affect the framerate.
Other than that I'd avoid doing it for something specific to one project. Others do it, but for me it's more tedious.
I'd recommend playing around with the sdk yourself though, it's fairly easy. I'm not sure it would be simpler to make your idea with a plugin than events though.