In this globalized world in which the internet connects civilized places to each other, I think the 5 year rule is outdated and unnecessary.
Foreign teachers in China can keep abreast of the evolution of standard English by listening to international broadcasters online and by using Skype to communicate with their friends and relations back home.
I wonder if other EFL countries have a rule like the 5 year rule as part of their visa laws.
I also think the Chinese government uses this rule as an indirect way of trying to discourage foreigners from staying in China for long periods of time. It seems like we are distrusted by the government here in the PRC!