Citation2, you may not agree but in my opinion this is how Airbus intended the procedure to be applied.
From the AFM:
"Apply 50 % N1 on both engines with brakes on.
Note: Brakes may be released so as to perform a rolling takeoff.
If crosswind at or below 20 kt and no tailwind: Release brakes with stick half forward. Apply thrust up to the FLX/TOGA thrust whilst maintaining stick half forward up to 80 kt."
At the end of the day they leave it up to you to decide by adding the note. However, they have made the brakes on until 50% the "standard" takeoff procedure but still left it up to the crew to make rolling takeoffs when required for operational reasons, for example when cleared line up and immediate takeoff due to traffic on final.
The most important thing in my opinion, brakes on or off, is to make sure engines are spooled up properly and stabilized at 50% before setting FLEX / TOGA. I've seen a few colleagues doing, what I think is a bit too sporty takeoffs, where the spool up towards 50% was slightly uneven and where they've not waited long enough for engines to catch up with each other before applying FLEX. Result is a very wobbly and unstable initial takeoff roll.
CP