Also consider that with speedbrakes stowed (flush) any roll demand generates some deployment of spoilers - on one side only, whereas with speedbrake deployed (say, halfway) a roll demand results in partial retraction of spoilers on one side and further deployment on the other. I'm sure the whizzo computers adjust the effect to some extent, but the added sensitivity is still noticeable, especially in turbulence with the autopilot working harder to maintain wings level. The book says 'not reccomended' but it is not a prohibition, and in certain conditions it may be the better solution (as opposed to early gear extension for instance).