What percentage of physics PhDs leave physics? What percentage of physics PhDs leave physics to become quantitative analysts, work in computer science/information technology or business? Is physics that bad that so many people leave? Was it worth it?
 A: In Germany the average age of a professor when he/she gets tenure is 42. A professor retires at the age of 65. If you add that the average professor will have one or two students per year finishing a PhD, you can figure out the percentage of physics PhD that can get a tenured position yourself :-)
As a physicist you are trained to do basic research, and there are almost no positions for doing basic research outside of academia, that's how the system works. So the question why one should study physics is legitimate. According to my own experience, physicists are employed as generalists when no specialized engineers are available.  When a company needs a software developer or an electro engineer for example, they will sometimes hire a physicist instead of someone with the matching degree, because "a brilliant physicist is able to learn the necessary skills on the job, so in the long term it is more profitable to have a brilliant physicist instead of a mediocre software scientist".  
A: Not pursuing an academic career does not necessarily mean 'leaving physics'. There are loads of physics PhDs working in industrial R&D. True, their careers don't focus on finding the Higgs boson, but there is a lot of interesting and rewarding work to be done in applied physics. And yes, occasionally industrial scientist scoop up a Nobel. 
A: A google search would be more help to you. The AIP has a page.
More people choose to do a phd in physics then there are tenure spots. Competition is high and monetary rewards not that hot. If one is a real physicist, one hangs on because of the physics interests not the career and yes, it is worth it.
Update after comment on the worth of hanging on:
A real physicist is like a real musician, painter, writer. He/she is in the field for the creativity outlet to his/her abilities. 
A: Just to note that people could leave research because they may not have "research grant funding" skills or aptitude.
A researcher who has invested a few years in a sub-field X may become almost a world expert in an aspect of X  - maybe in one of the experimental/computational/calculational aspects. Unfortunately fashions change in science, in Physics perhaps because experiments are starting to disprove the assumptions/predictions of X.
So our researcher finds no more money to work in sub-field X. Our researcher now faces some stark choices: adapt the experimental/calculational techniques to a popular field like Y; physically relocate to an Institution that still pursues X; or develop a new sub-field; or leave research.
If our researcher stays on and hopes to change sub-field they will now face a research grant procedure, which although not hostile to them, is hostile to sub-field X which becomes a liability from now on. Some individuals have the skills to negotiate this, some dont.
A: @Tim van Beek To look at it in another way, a lot of people get into physics with the aim of walking away from it after the undergrad, grad or phd. For instance, people may choose to go into environmental science. And a lot of physics phds go into biology because the problems there today are much more interesting than they are in physics. And lets face it. Biology has a lot of catching up to do!! :P
