To clip or not to clip???

Should chickens have their wings clipped? My four are very happy in a decent sized 12 x 6 run plus the 6 x 5 garden shed and they have a 6foot high x 6x6 section inside the 12 x 6 area where I have stuck some perches which they go up on some time. They are in there all the time, my mum in law says they should have their wings clipped... should they?? If so why?? Cheers for the advice....

Thought that made sense but didn't like to argue with the mother in law. Thanks for the help I can refer her to your email, in fact I might print it out as proof... she does make a good Sunday lunch and wouldn't like to miss out....