I agree completely.
The most common pro-tenure argument is that it protects academic freedom. Once a professor gains tenure, the thinking goes, he or she can say anything without fear of being fired. Academia thrives on the circulation of dangerous ideas.
The problem is, for every tenured professor who's liberated at age 40 to speak his mind, there are dozens of junior professors terrified to say anything the least bit controversial, lest they lose their one shot at job security for life. Academia relies on young scholars to shake things up. Yet tenure incentivizes them not to. Instead, it rewards students who follow in the footsteps of the elders whose favor they will require when the day of judgment arrives.
Tenure is being phased out in British universities, and my friends there report that this has made no difference. Professors without tenure become regular civil service employees, whom it is difficult to fire as long as they are doing their jobs; and why would we want to protect professors who are not doing their jobs from dismissal?
The only argument I can see for tenure is the reverse of the usual one, that is, because it provides at point at which universities can get rid of people who just aren't working out. Without the ritual of the tenure review it might be very difficult to ever fire any professor at any point in his or her career.