I think Sir Donald was probably referring to his company limit which, like mine, is Flap 5 with speedbrake. Seattle's limit is Flap 10 and I've utilised this in the Sim with a medical emerg to facilitate a straight in. Speedbrake can be very usefull with Flap when coming down a slightly steep glide such as Malaga 13. It keeps the speed in check and avoids an unecessarily high throttle position that deploying the gear would lead to. Snag is if you go from Flap 5 to Flap 15 and gear and forget to stow the brake you'll bust the limit and have some paperwork to file. Obviously if its not enough then drop the gear. Some guy's just don't like using brake with flap so will always use the gear. Nothing wrong with that and at least it avoids the trap above albeit its not very fuel or noise efficient.
Using the speedbrake at high speeds will put more stress on the airframe but if your in a moded aircraft there is no longer any restriction on it. If you plan ahead wisely you should be able to avoid the need for it. If its required then use it but it should be the exception at high speed not the norm.
Those who say its either down or at the flight detent are correct as the aircraft employs differential spoilers any intermediate positions can cause rapid roll rates. I think boeing document that as well.