Sam never gives up on anything. So maybe it's intended to really drive the point home, showing us that if even Sam is admitting defeat, then it really, really is completely futile. After all, it's been months and the only thing he's found is Ruby and her vague promises. The show is definitely emphasising the hopelessness of Dean's situation in the last few episodes. I still tend to believe that this is to emphasise the magnitude of the miracle when it finally comes *G*

Except of course that Dean will already have hellhounds chewing on him by then, so as to amp up the tension even post-salvation, because this is Supernatural, and even miracles don't come for free, and nothing is ever simple or straightforward! That's my prediction, anyway, which will stand until new information becomes available...