i completely agree with you. i am a PhD student and this idea doesnt make any sense as doing PhD doesnt mean getting a degree, it takes 3-5 years of a much qualified person and basically what you learn is implementation of each and everything what you have done so far. PhD develops candidate's thinking process and how to put that in practical. i think if you will start having expiry date on PhD then you got do that with all undergraduate degrees as well, as knowledge (taught in UGs) does expire as time goes on but my general perception is that all PhDs remain quite active and try to keep themselves upto date with the curreent research and developments....
