Up until the middle of last week, I was working a full time job and doing PhD work ( via distance learning). It was a struggle to do the PhD work along with a job, but I do think it helped to keep a balance and perspective on things. I can see how doing a PhD full on can create the sorts of issues people are talking about--you get isolated and over-focussed, and that can lead to all kinds of problems.
While many might disagree, I think that SOMETHING besides a PhD in a routine, be it paid work, volunteer work, or even a regular exercise class you take at a gym, making sure to keep some kind of contact with people who do not understand your PhD and therefore you have to talk about other things, is essential to good mental and physical health.