I disagree with the last two,
I think if it was a one off you should do it, provided you are insured to fly the aircraft in question. It would show you are keen and willing to gain experience, but I wouldnt advise doing it all the time for free. If I offered the chance to a new CPL pilot and they started dictating terms and conditions and wanting paid, It would seem very arrogant to me and i would tell them to get lost...
BC.