CAPT OF AN AIRBUS 321 OVERSHOT INTERMEDIATE STAR ALT FIX DUE TO BEING DISTR BY TFC WATCH; REPROGRAMMING FMS WITH NEW RWY ASSIGNMENT AND NEEDING TO EXPEDITE DSCNT JUST PRIOR TO THE REQUIRED ALT FIX.

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OPERATING AN AIRBUS 321 INTO LAX; ON THE MITTS 1 ARR; WE WERE CLRED TO DSND ON THE PROFILE AND CROSS 'AT OR ABOVE' ALL FIXES. AS WE WERE HIGH ON THE PROFILE TO THE RWY (RWY 24R) THE ACFT WAS IN AN IDLE DSCNT WITH SPD BRAKES EXTENDED. JUST PRIOR TO XING THE MITTS INTXN; WE PASSED THROUGH 10000 FT AND LEVELED OFF AT 9000 FT UNTIL JONZZ INTXN. AFTER JOINING THE FINAL APCH COURSE; A NORMAL LNDG WAS MADE. MY ESTIMATION OF THE CAUSE OF THE XING ALT ERROR AT MITTS WAS THE INITIAL NEED TO EXPEDITE DSCNT TO CROSS CIVET INTXN AT 14000 FT. HIGH WORKLOAD LOOKING FOR TFC AS WELL AS REPROGRAMMING RWY. ASSIGNMENT INTO FMC COUPLED WITH FAILURE TO RESUME NORMAL DSCNT RATE CONTRIBUTED TO THE MISSED ALT. HAD THE INITIAL STAR BEEN LEFT UNCHANGED; AND LOWER ALTS BEEN GIVEN IN A TIMELY MANNER; WORKLOAD WOULD BE VASTLY REDUCED; THEREBY FOSTERING MORE VIGILANCE BY THE FLC. AS WE ALL KNOW; THE ADVANCED FLT DECK AND FMC SUCH AS THE AIRBUS HAS; CAN ONLY REDUCE WORKLOAD AND INCREASE SAFETY IF IT IS ALLOWED TO DO SO BY LEAVING THE FLT PLAN AS WAS PROGRAMMED AT PREFLT UNALTERED. LAX AND SOCAL APCH HAVE A WELL EARNED; INDUSTRY-WIDE REPUTATION FOR MULTIPLE STAR/SID AND APCH/RWY CHANGES. WITH SO MUCH TFC IN THIS CORRIDOR; IT IS MUCH SAFER TO MAINTAIN SEPARATION THAN TO BE 'HEADS DOWN' MAKING ATC REVISIONS TO THE FLT PLAN.