While it is a very useful feature, there are some flaws in the ability to move cities - not least of all being that you can only move your Capitol.
I agree with the facility, people don't often decide on what alliance to join until they have been here for some time and often the alliance they join is some distance away from them.
Having said that, it would be interesting to have a facility that would allow you to move
all towns/cities although I think that to prevent it being exploited, it should be limited to say once per year.
In addition, again to prevent exploitation, I think that if a player is a member of an alliance that is at war or has come under attack, the facility should 'freeze' until either the war has been declared over or the player has not been attacked for 28 days. If this was adopted, there would have to be some exceptions I guess as a war should not prevent a new member to an alliance from moving towards their alliance area, and thus the war zone, only prevent them from moving away from it.