When you use only cotton, it will behave relatively predictably. You don't want one part to shrink differently than another or one touchy fabric discombouber in the dryer. Mixed fabrics are less predictable. I have used some cotton blend pieces in the past but I try to use all cotton. Love the way it handles over time.
Sometimes the discussion about fabrics is more about the quality and price of cotton (LQS versus WalMart) and some people prefer the better brands while others say to use what you want/have.
As for the thread, I have seen hand-quilting threads, but since I don't hand quilt - I never paid much attention. I assume that the thread can withstand more tugging than some of the others.