A PhD is basically a hunting license for the very few high level science jobs available and the fewer still that are applicable to your training.
First off we have the issues with the PhD program itself.
http://liv.dreamwidth.org/389934.html
This article although no written for Science PhD programs in the US in particulary describes exactly the situation I saw in grad school. It is now a 5-7 year program where you need to find a mentor who is actually interested in helping to develop you rather than use you as a cheap lab tech or Teaching Assistant. There are a lot of sociopaths with tenure who will use and abuse you.
Then as I indicated above there far far far more PhD scientists than there are jobs or grants for. It is like game of musical chairs only there are 100 players and 3 chairs. We then have scientists who sacrificed their 20’s and 30’s busting their rear working gruelling hours and training only to waste away in post-docs and end up unemployed or an adjunct professor or making poverty wages teaching community college part time.
The situation is really quite tragic and a horrible waste of some of the brightest minds in our country. Please don’t get brainwashed by a romanticized and propagandised view of a scientific career. It is not pretty for most people. There are a lot of destroyed lives.