In what way is Jurgis rare among criminals, making him a valuable friend for a man like Jack Duane?

"And so, when he was turned out of prison again, without a penny in his pocket, he went straight to Jack Duane. He went full of humility and gratitude; for Duane was a gentleman, and a man with a profession--and it was remarkable that he should be willing to throw in his lot with a humble workingman, one who had even been a beggar and a tramp. Jurgis could not see what help he could be to him; but he did not understand that a man like himself--who could be trusted to stand by any one who was kind to him--was as rare among criminals as among any other class of men."