Let me be clear for others who come across this because I'm completely sure that since you've been a student and now work in the system you know where I am coming from as a student. Surely there will be people who read this who have not had the 'pleasure' of being graced with the "bad kind" of tenured professor.
In my college experience, which has only been one year, I've met a few types of professors.
There is the kind who LOVES teaching and who is amazing at it. This is the only kind of teacher that I'll give a real life example of. If you're going to NJIT and need a math teacher you're going to want to look for these two names: Jimmy Hayes and Kenneth Horwitz.
Both don't only want you to learn but they also want to be the one who teaches you. If I could have every professor I meet conduct themselves like this I'd have no problem with tenure. And that's coming from someone who failed the class taught by Hayes. The reason I'm giving such praise to these professors is there is absolutely no way my failing grade was their Hayes fault, it was my own (I had some problems going on in my life at the time).
They spent all the time they could preparing us for the exams and also gave us points back if we did something that was incorrectly graded and we could prove it was right.
Next there is the kind who "believes in the system". They teach because they enjoy it, they are usually charismatic and good at teaching but that isn't their main focus. They either do this while engaging in research or other projects to help the school or further their field.
I have no problem with this second group. It is again someone who puts time and effort into their job. Someone who cares about their work. It's altogether someone who is furthering humanity while not harming others around them.
The last professor is someone you don't want. It's the professor who is tenured. That's how all of us students refer to them since these seem to be the only kind of people, in the eye of a student, who actually has it. They are the professor who comes in 15 minutes late to class, consistently, refuses to answer emails or questions, and leaves randomly through the semester.
I've actually had one of or physics professors come in on the first day 15 minutes late. He started the lecture (for the class that was meant to be taken along side of Calculus 1) by saying "So does everyone here know how to take the derivative." When everyone is confused, he does a 10-second-basic drawing of what it was. Asked if anyone had any questions. He scanned the room full of raised hands and said "Ok" then went on with the lecture.
This happened 10-15 times across the semester.
Another incident was that the physics department cobbles together a "sort of practice exam" for each test. I found this out from a friend as my teacher had never talked about nor handed this exam out.
The last story I'll share of this horrible professor took place around the time of the 3rd exam. I took the exam and went in the next class to get my scores only to find he had absconded with the only copies of our exam on a "business trip"; we had to wait 3 weeks to get our exam scores. (I had given up at that point and started randomly answering on the exams. For this test I got 17/20 placing me in the top 20% of the test takers.... that might tell you a bit about our exams)
So this is my problem with tenure. It's often the WORST of the worst you notice have it while the best of the best struggle to get it since it seems they aren't one to shut up and play ball with the stupid processes needed to get it.