An A320 pilot flying the LAX SEAVU TWO was prevented by Company procedures from using the managed descent mode and after manually setting thirteen step down altitudes missed the fourteenth and got low on profile.

Narrative

We were flying the SEAVU2 arrival to the ILS25L at KLAX. SOCAL cleared us for the ILS while still WAY out on the SEAVU arrival. The fact that they do this now is a HUGE improvement over the 'old days' when they gave multiple descents to the approach. The problem arises in that my airline still refuses to let us use the 'managed descent' mode of the autopilot. There are a total of 14 intermediate altitude restrictions on the combined STAR and ILS. If we could use 'managed descent' we would enter the lowest altitude restriction; press the managed descent knob (altitude knob) ONCE; then monitor the autopilot as it followed the descent profile. The way my air carrier makes us do it is to set ALL of the intermediate altitudes as we approach them; hoping the FMA doesn't go to ALT* before we're able to set the next altitude. This results in 14 'settings' of the altitude knob and usually as many pushes/pulls of the knob during the descent.Tonight I got 13 of them right; but managed to get about 180 FT low at GAATE. It sure would be nice to be able to use all the features at our disposal; it certainly would have prevented this altitude bust.