Kev - nice quote from the paving expert it's a great ref. web site!
I am of the mind that sealing has little effect on weeds, and moss, for weeds just look around and see the damage they do to Tarmac and cement, a bit of urethane sealer a few microns thick won't stop them coming through. And moss spores will just lay on the surface and still spread.
I will seal if the customer really wants it, but my advice to them is to maintain the drive/patio by washing over it in autumn and spring with warm soapy water and scrub with a stiff yard broom, keep it clear and tidy.
Darran