Academic Tenure
In my opinion,  the concept of tenure, be it for individuals in universities, research institutions, or government, is here to stay. Tenure represents a major factor in assuring the security of qualified academicians or researchers and also the professional quality of the institution in which he or she is employed.  There is,  of course, the unpredictable negative feature of "burn-out" and intellectual lassitude among some tenured staff members who must occasionally be encouraged to depart by enforced early retirement or by the imposition of restricted facilities.   When I 
first arrived at the National Institutes of Health in the early 50's,  a number of the Institutes had practiced a more liberal interpretation of promotion and tenure than was of long-term benefit to the productivity and general ambiance of their laboratories. The  "dead weight"  that had accumulated constituted a severe burden on both laboratory space and financial outflow, and only after a number of years was this alleviated through the process of attrition due to retirement or to migration to other sites of employment.
I might add, parenthetically, that in the case of the National Institutes of Health,  the situation has now gone perhaps a bit too much in the other direction, and the current budgetary problems facing the world of research and academia are resulting in increasingly restrictive employment and tenure limitations that are causing considerable anxiety in many of the subdivisions of the NIH.
The major function of well-defined tenure rules, in most universities at least  (and the same is probably true of primarily  research institutions such as The Weizmann 
Institute of Science) is the stimulating and, unfortunately, competitive influence imposed on the younger members of the professional staff.  The intensity of such pressure is a function of the rules within any given university.   When I was at Harvard in the 40's and early 50's, the time limitation was,  as I remember, eleven years for the "up and out" rule applying to new young members of the faculty.  This length of time has,  I believe by now, been shortened to something on the order of six years.  At The Johns Hopkins University where I am now employed, this length of "trial time" is about the same.  An assistant professor at Hopkins may hold this rank for no more than seven years,  and if he is not to be promoted by the beginning of his seventh year of service, he must be given notice before the end of the sixth year.  Although an occasional associate professor may be given tenure at that level,  individuals at this rank are generally expected to receive promotion to the professor level at the end of six years or move elsewhere.
The influence of the tenure rules on the security of a young academician or researcher varies enormously from institution to institution, and particularly from country to country. A country such as Israel, for example, has very special problems,  and all of us who have been associated with members of the Promotions Committee at The Weizmann Institute know the level of trauma that is felt not only by the young scientists under consideration for tenure, but by the committee itself.   In Israel -- a small country with limited opportunities -- the alternatives for a young person who is not able to achieve tenure after the six-year period are very limited:  such persons cannot easily move to any one of a large number of other institutions as they can, for example, in the United States or in England or in much of Europe. This leads to a migration of quite good people from an academic environment to an industrial one.  This is not, in itself, necessarily cataclysmic since many industrial positions involving research can be interesting and highly creative.  More serious, however, is the tendency for a young scientist or engineer who has not received tenure within the allotted time to join the ever-growing fraternity of "yordim" who are tempted by the scientific fleshpots of the West.  If there is one thing that Israel needs, it is to keep its bright, educated young people within the country and to attempt to create and stimulate institutions or industries that can utilize their talents and maintain an important critical mass of technically-trained individuals within the country's economy.
There are, of course,  as in any branch of society, persons who are simply not cut out for academic or scientific work and such people, when refused the reward of tenure, might find themselves directed towards other forms of human activity -- I might mention the teaching of pupils in schools below the university level -- and, once again, activity in industrial organizations that require sizable staffs that need not necessarily be made up of creative leaders.
I think we might well take The Weizmann Institute as a good example of a  place where tenure consideration, after a limited length of time, is absolutely essential so long as budgetary considerations are of as great an importance as they are now,  and when laboratory space and required equipment must be apportioned out carefully to highly qualified individuals. There seems to be no other way of insuring continued excellence within the Institute short of discovering oil on the premises.