Some interesting points. Certainly a very good idea to run the system for about 10 mins once a week all year through and as MD says run it on hot to kill off any bacteria. In winter you can use the system for demisting so you would probably be doing this anyway.
Also it is true that the system fitted to the Focus gives few problems, I have been told this by several air con specialists. Whether this applies to all Fords I don't know.
There is no reason why a recharge service should cause problems unless the tech has done something wrong.
Now the thorny issue of recharging. It is widely accepted that car air con systems will lose from 5% to 10% efficiency a year (depending on who you speak to). From the day it is filled at the factory it is beginning a very very slow decline. I see parallels here with something like shock absorbers on a car - the decline is so gradual that people don't notice it until they need new ones (due to an MoT maybe) and then they notice the difference in the way the car feels.
My cheap runabout car is a 2007 Focus Ti-VCT, just on 26000 miles and 3.5 years old. I've just had the aircon recharged for £47 at a trusted family-owned garage. They extract the gas and measure the difference between what is in the system compared to what it should be. They vacuum test, clean and recharge with recycled gas and new compressor oil. Mine was about 25% down.