B757 Captain experiences asymmetric spoiler extension in flight causing the aircraft to roll left. Upon arrival Maintenance elects to defer the auto speed brake function and send the aircraft out.

Narrative

While on the descent; I extended the speed brakes to make a crossing restriction. We were at 27;000 at 320 KTS assigned. The autopilot was on; and as the speed brakes extended; the aircraft used 1 unit of right aileron to fly straight and level. When the speedbrakes were retracted; the ailerons returned to neutral. As there was history of this; I disconnected the autopilot and again extended the speedbrakes. The aircraft rolled quickly to the left; and right aileron was need to stop the roll. We sent a report to Maintenance. When we got to the gate; the mechanics who met the airplane said that Maintenance was going to defer the auto speed brake function and send the aircraft to LAS; since they had a long runway. Three major issues with this thinking.1-This is a chronic write up; with each time; ground test; no faults noted; as the response.2-This does NOT address the write up. Spoilers in flight and on the ground are two completely different issues.3-An aircraft that is headed to LAS may have a situation arise which requires an immediate landing at nearest suitable airport and the speed brakes may be required due to landing on a shorter runway. The MEL was designed to get an airplane back to a maintenance base to be fixed; not to fly from a maintenance base at the convenience of the router!