This topic is funny because the 5 turn restriction didn't exist in any form when Civ4 was still new.
Then some Roman posted a deity game where she constantly switched civics to keep herself in perpetual anarchy, thus getting rid of all maintenance costs, while chopping swarms of praetorians to roll over everyone and capture cities for a super early conquest or domination win, I don't remember which. Some mod saw the thread, freaked out and blew a gasket in it, and then BAM, in the next installment (I forget if this was a patch or Warlords) the 5-turn restriction was there. We never even got to abuse the late game possibilities of not having that mechanic till now because some cheesemonger ruined it for us.