I don't know about bloody mindedness. Most scientists focus on one small idea simply out of necessity: the age of individuals coming out with bold, broad theories is largely over, for the time being at least. There are limits on what one person can do or say, and science today is far more technical, and far less intellectual than perhaps previously was the case, being a much more collaborative, labour intensive, and, dare I say it, boring endeavour.
Take the search for the Higgs boson, one of the biggest recent scientific stories. A nifty idea, yes, but all the fun, imaginative and clever theorizing was done back in the 60's. What's happening at CERN is basically a very grand take on tedious hypothesis testing involving smashing crap together and seeing what happens. All that effort and time spent trying to validate someone else's idea, only to watch them being lauded as a visionary for ever more whilst your fame rapidly diminishes to that of the average lab monkey keener.
Anyway, a lot of people get into science filled with enthusiasm only to then realise that their field is so cluttered and that so much is already known that they're basically going to be spending their lives studying things like 'the Saville ramified p-correspondance of bleb factors in a three point pseudo-Eric/Ernie system'. No sane individual wants that as their lot in life, but by then it's far too late: they're already overeducated and underskilled to the point of being unemployable. By then, the system has them.
So, to get back on track, what you see as the similarities between professors and hobos isn't bloody mindedness*, it's a shared abandonment of hope tinged with bitterness at the realization of just how futile one's life is . Walk down a department hallway late in the evening, all you'll hear is quiet sobbing and cries from the more thoughtful members of staff consisting of 'why couldn't I be something useful and interesting, like a hatstand or someone who designs fliers for underground dubstep nights'.
*Also, I'm not entirely sure people sleep rough out of sheer bloody mindedness...