I'm not well versed in the details of iOS development. Why is this inconvenient during development? Is the bitcode portion copied to your device and is a slow process?
Some people like to check in precompiled versions of their dependencies. Others like to distribute precompiled versions of their frameworks or libraries. Larger binaries aren't great for either of those cases.
Larger files to store and upload to iTunes Connect mostly. I don't think bitcode is ever transferred to development devices.
I guess there is also a risk that the compilation process generating 3x larger files could also be slower as more work are being done - but that's just speculation.