The best explanation behind the appeal of a PhD, to me, is expressed in a comment on an Ars Technica story:
Someone - an eccentric billionaire, say - contacts you and offers you three or four years on a paradise island, all expenses paid. Accommodations will be sparse but sufficient, and you'll have lots of time, excellent connectivity and lots of resources to pursue whatever project strikes your fancy. Of course, it's limited to the agreed time; once the time is up you're off the island and back in your home time to fend for yourself again. [...] The point is, doing a PhD isn't only - or even most - about what kind of career it will get you.
Of course, I'm an optimist who's not interested in earning money just for the sake of it. By all means do a PhD if it does lead you towards a better career, but I recommend doing one for totally different reasons: learning about who you are and what you like to do. I had no idea who I was when I started my PhD, and the long, long leash given to me by my supervisors has allowed me to explore such things about myself.