I brought a sleeping bag with me on my lst trip and I used it only 2 nights of the 9 I was away...Most hostels DO NOT let you use them in fear of bring bedbugs or lice from another hostel. You are normally provided with sheets and a blanket by the hostel, which should be changed every day.
My sleeping bag is very small in size, but very warm and long enough that my 6'4" frame can fit in upto my neck. It doubled as a pillow on long train rides, and when I slept on the floor of the ship, i stuffed the cover with clothes and made a pillow. Fully rolled up, it maybe weighs 2-3 lbs, so it was not heavy to carry. It fit right inside my pack, so not to be exposed to the elements. I was glad I had it with me, but only because it was so small, if I had of had a big sleeping bag, I might not have liked carring it around so much.