When I first studied the Bartok, with Marjorie Tyre, I was told not to roll chords in 20th Century music unless indicated. So I have never rolled the chords in the fourth movement of the Bartok. However, many of the orchestral study books instruct harpists to roll those chords, and of course it sounds very lush and beautiful when played that way.
However, Bartok's music is so angular and full of accents that it feels wrong to me, and unstylistic. I suppose I am "old school," as it seems most harpists do indeed roll the chords. I should add that no conductor has ever asked me to roll the chords, including Eschenbach, the music director of the National Symphony, who is a bit of a micro-manager. The same is true of Leonard Slatkin and Ivan Fischer. We played the chords unbroken and received no comment from the conductor.
I asked Dotian Levalier, principal harpist of the National Symphony Orchestra for 44 years, for her comments. She told me she first played the work under Dorati, who was a colleague of Bartok. Dorati told her there should not be the slightest break in the chords, and we assume that was Bartok's request.
I am attaching an audio file I found of Dorati conducting the London Philharmonic and, indeed, the chords are unbroken.
Download 1-04 Concerto for Orchestra, Sz. 116
