So I've been meaning to propose this. Here's a first stab
Whether there's any chance to get it approved by the group I don't know.
*) Multiple render targets are super awesome! Lots, in fact most modern graphics applications use this feature extensively. Supporting it will go a long way to helping adoption of WebGL with more impressive apps.
*) It's not really a new feature. No new APIs are added. It's similar to texture fetch in vertex shaders where the max allowed is allowed to be 1.
*) It's relatively easy to implement
*) It doesn't raise any extra security or validation issues that I know of.
*) Some mobile hardware only supports 1 target.
I don't really see that as much a valid argument in that some day this feature will be supported one way or another (WebGL 2.0?), at that point the same issue will exist, some hardware will support 0, some 4, some 8, some 16, etc.. Apps that use multiple render targets will have to have fallbacks for hardware that doesn't support the number they ideally need. So, why not just expose it now?