The npot textures extension was indeed not supported out of concern about lack of support by mobile hardware. I don't feel very strongly either way, but at least nothing changed and the approach here has always been consistent.
So let's move this discussion forward in two ways. First, please identify those extensions which enjoy broad support on mobiles so we can forget the scaremongering over those.
Secondly, For the other extensions, I'd like to point out the following three things:
1) Them being extensions already satisfies the "accidental usage". Functionality should only be exposed if explicitly asked for to avoid accidental usage.
2) A device support matrix should be created enumerating where you can use what extensions so developers can make an informed choice easily.
Lastly, to the vendors concerned:
- If you still worry about accidental feature use, the sooner you make your devices support WebGL, the less you need to worry about *your* devices (I would have an iOS and Android device for WebGL testing right now if they would support it).
- Maybe if a feature you don't support proves exceedingly popular (like say oh multi render targets or npot textures), it's time to think about supporting it? Nvidias tegra seems to want to support a whole lot of things, I'm sure you want to keep up.