That depends. Back in the day, in my research institute, it was common practice to print out the papers that should be read thoroughly - after skimming them on a Laptop screen.
Not all ones not mentioned in this thread yet. A secondary 19" monitor ties you to a desk. A tablet would allow reading wherever people typically read paper printouts (in the train, while sitting on a couch, etc.)
When I have a complex paper to read I will spread it out on the floor. A single page at a time is too limiting, I want to be able to see page 5 and page 12 at the same time.
It was nice to be able to jot things down in the margins (math, not just notes). Provide yourself the couple steps the author skipped (to show how clever they are).
There's only a certain number of papers you can carry around and flip through like that. With a tablet, it can store hundreds or thousands of papers without getting any more unwieldy.
