Even if it's opt-in, we still pay a price. Complexity has its costs.
It means my browser is more bigger and more complex. It broadens the attack surface exposed by my browser, perhaps even if I don't opt-in. It dilutes the efforts of the Firefox team. It introduces new ways for browsers to be subtly incompatible. It further raises the barrier to someone making a serious browser from scratch.
Also, are we guaranteed that these features will be opt-in? There were serious security issues with WebGL (predictably), and I don't think WebGL was opt-in.
WebGL was never opt-in and everybody familiar with GPU programming saw the issues coming from miles away. It was an idea that was about as stupid as Java applet sandboxing.
It means my browser is more bigger and more complex. It broadens the attack surface exposed by my browser, perhaps even if I don't opt-in. It dilutes the efforts of the Firefox team. It introduces new ways for browsers to be subtly incompatible. It further raises the barrier to someone making a serious browser from scratch.
Also, are we guaranteed that these features will be opt-in? There were serious security issues with WebGL (predictably), and I don't think WebGL was opt-in.