Early 737s did not have the pin and isolation valve for the nose wheel steering. So in that case the A pumps were selected off. The selenoid valves that close off the A pumps are power to close/isolate. When switching power sources (bringing the generators online) the concern is the A pump momentarily pressurizing A system.
So, we do it because that way it is standard with the early acft and also provides a back up to the pin. If you were at a carrier that never had old acft, they may not do it that way.
Lock out pin reliability? Should be good. But right now A 320s have some sort of problem with this causing un intended nose gear movement even with the lock out pin installed. So right now to start an A 320 with the towbar connected we have to turn off the BSCU which removes the hydraulic power to the steering. New plane, old problem.
So somewhat tradition, somewhat extra insurance. The old aircraft are long gone but it does provide extra protection.