Please can you indicate exactly which kind of mouse you use?
Actually, I have already done numerous breaking changes whereas JogAmp's Ardor3D Continuation will have its very first stable release next year, the mouse buttons will go on being managed that way even though it's not very flexible.
Maybe I can replace the RuntimeException by a simple warning but I'll have to modify all callers as they don't support receiving a null value :s
Keep in mind that the mice with an hidden mouse button using 10 as an identifier are probably very rare. I've found a mouse with 6 buttons, it's really difficult to find a mouse with more than 10 buttons, there are a few gamer's mice with 20 buttons.
It's very important to be able to wrap all events of the toolkit into events of the engine so that it's possible to write some code working on several toolkits without any major change.
I'll probably support mice with 32 buttons.