Air carrier taxiing for departure at LAX on the South side; expecting Runway 25R; is assigned Runway 25L and cleared for an immediate departure; the reporter suggesting ATC should have provided better planning information.

Narrative

Runways in use for takeoff Runway 25L/25R LAX. Our aircraft was on the south side of the airport. Normally we depart Runway 25R; which gives time for normal pacing for checklists; etc. Ground switched us to Tower frequency due to short taxi; to hold short of Runway 25L. As Captain; I interpreted this clearance to hold short of Runway 25L as 'a hold short;' to expect to cross Runway 25L for departure for Runway 25R. At no time did Tower tell us to expect an immediate departure on Runway 25L. Luckily; all major checks and taxi checklist had been completed. Our aircraft approached the hold short line. We were cleared for an immediate takeoff; traffic on a 4 mile final. My company's SOP has the aircraft come to a complete stop in the runway to complete takeoff checklist and perform a static run-up prior to brake release. As Captain and in the interest of a safer operation; I elected to perform a rolling takeoff. Tower was telling us to expedite; traffic on less than a 2 mile final. Had I not done this; there would have been a multiple aircraft go-around at LAX! Tower personal created a very unsafe situation; they need to let air crews know! The terminology; 'Hold Short Runway 25L; expect immediate takeoff; no delay' Tower personnel also need to consider the possibility of a rejected takeoff. Heavy aircraft such as B747 deserve increased margins; especially at LAX; with high density operations in progress.