well reasons it sucks

1.There is no attack in FFX that can deal over 9,999 to ALL your party members that cannot be predicted or defended against.

2.BHPL make your Celestial weapons crap. Weapons like the Caladbolg and World Champion use their full potential when your health is full, and keeping 99,999hp is annoying and wastes precious turns.

3.There is no item that can restore over 9,999hp...so it's best to keep your HP capped at that amount to save space on your armor and on your sphere grid.