I just ran some tests and the Extra outputs do behave oddly.
If you set .tif as the Save as Type in the Extra Output images, you still get .exr except for:
tgAlpha
tgRgb
tgSampleRate
tgSurfDepth
(on build 4.5.56)
which can output as .tif and according to Nuke are either 8 or 16 bit fixed depending on the .tif options selected.
That seems a bit random.
I'm guessing it's the same code for the Save as Type selector in the Extra Output images, and those Render Elements need to output float which is why they revert to exr?
I don't use anything other than .exr so it's not a problem for me, but maybe a bit limiting for anyone who doesn't have an .exr compliant app who might want to check out the render elements.
What would be very useful is a 16/32bit switch between the data and color Render Elements - 32 bit is a bit overkill for the color elements (surface Direct, alpha, RGB) whereas the data elements would always benefit from being 32 bit.