I don't think it's so unreasonable the way it is, but I can see the benefit in allowing for screenshotting.
However, it would pull things out of line with what readPixels will give you back.
This is also potentially non-trivial on Firefox OS.
Why do devs not just use pDB=true if they need this functionality? If they really do need the bit of extra perf (which should be quite uncommon), why not add an option to do a screenshotting frame render?
I can see this making life easier for some, but I believe it may prove to be a headache to implement in some cases, where most apps can use pDB, and the rest can do their own screenshotting?