It's simple mate the role can be altered any time thus PF can become PNF in split of the second. Bear in mind that being PF doesn't mean being in charge, PIC is always in command and can take controls whenever deemed necessary. Now picture this: F/O is PF, he/she takes off and for whatever reason let it be technical or otherwise the PIC decides or has to take controls and there's a need to perform checklist in QRH. Now what? You stepped onto a slippery ground, how will you pass the QRH to the F/O and fly at the same time. Remember, one has to be on controls at all times. Big deal, probably not, big loophole to be held liable, probably yes.
