Break HP Limit is the most overrated ability in the game. More HP, something
bad to it? No, not at first. However, when you start fighting, you will see
that having more HP won't actually help you. Combined with Auto-Potion, 9999
HP is the best to have.
To explain this, I copied this list. This was originally created by Theoden and
credits go to him.
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BASICS
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• You can't heal more than 9999HP to everyone at once, except with Rikku's
Overdrive. Items are capped at 9999. Trio of 9999 and Pray will fully heal a
party that doesn't use BHPL.

• Stoic/Comrade will give slower Overdrive gain with higher max HP.

• Proportional damage attacks (e.g. Hydraulic Press, Demi, Mega Graviton, Fangs
of Chaos, etc) will do more damage with higher max HP. With 9999 HP, you can
fully heal in one shot. A BHPL character would have to waste more turns
healing.

• Celestial Weapons for four of the characters will be weakened if they don't
keep HP topped up.

• You never need BHPL. There are no attacks in the game that hit the whole
party for more than 9999 damage that cannot be predicted, blocked, reduced
below 9999 with Defensive measures or avoided altogether. Auto-Phoenix takes
care of attacks that hit a single character.

• Break HP Limit is useless if you haven't activated enough nodes to go over
9999 HP yet. Auto-Phoenix is useful throughout the whole game. Having two
characters with Auto-Phoenix in the active party makes you almost invincible
against attacks that hit just one character, no matter how powerful.

• Auto-Phoenix restores 9999HP max, so BHPL characters revived by it will be
near . Using Full Life to revive means wasting more turns, for both the
KO'ed character, and the character casting Full Life.

• BHPL won't make any difference if you get hit for 99,999 damage or instant
Auto-Phoenix will. Powerful attacks will hit low Defence for 99,999
damage, so you need to raise Defence/Magic Def for BHPL to be useful. But if
you do raise them, you won't need BHPL, and they are easier to raise than HP.
Tanket and One Eye can be killed more easily and quickly than Ironclad.

• Getting Break HP Limit is tedious and time consuming.

• Collecting HP spheres from Ironclad is tedious and time consuming.

• Auto-Potion is good for maintaining full HP in some tough battles, but if you
have BHPL, it'll only keep you just above 50% HP.

ATTACKS THAT PENETRATE 9999HP
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Just to make things seems a little clearer, here's a nice little list showing
you what attacks will penetrate 9999 HP, even with max Defence or Magic
Defence. Use the legend at the bottom to understand what the number's beside
each attack mean.

• Karma used by Tonberries (1,~3,5)
• 10,000 Needles used by Cactuars and Cactuar King (1,5)
• 99,999 Needles used by Cactuar King (1,3,5)
• Regurgitate used by Geosgaeono, only used if character kills him while
swallowed. (5)
• Zanmato used by Yojimbo/Dark Yojimbo (2,3,5)
• Convergence used by Th'uban (1,2,~3,5)
• Ethereal Cannon used by Nemesis (1,4)
• Ultra Spark used by Nemesis. The one attack that requires Auto-Life. Although
technically not predictable, (2) applies, since you can kill him before he
gets 8 turns.
• Armageddon used by Nemesis (3,5)
• Energy Blast used by Dark Valefor (2,5) *
• Meteor Strike used by Dark Ifrit (1,2,5) *
• Hellfire used by Dark Ifrit (2,5) *
• Thor's Hammer used by Dark Ixion (2,5) *
• Diamond Dust used by Dark Shiva (2,5) *
• Mega Flare used by Dark Bahamut (2) *
• Wakizashi used by Dark Yojimbo (4) *
• Oblivion used by Dark Anima (2) *
• Delta Attack used by the Dark Magus Sisters (2,3,5) *
• Camisade used by Dark Cindy (1) *
• Passado used by Dark Mindy - requires proof even with BHPL (1) *
• Razzia used by Dark Sandy (1) *
• Obliteration used by Penance (4) *
• Immolation used by Penance (1,4) *
• Judgement Day used by Penance (2,3,5) *

1: Hits only one character, so Auto-Phoenix can take care of it.
2: Attack used predictably, so can be blocked with an aeon.
3: Does 99,999 damage, so BHPL makes no difference.
4: Can be reduced below 9999 with additional support (e.g. Defence/Magic Def
+20%, Cheer, Focus, Shell, or Sentinel).
5: Preventable (enemy can be killed before using it, or conditions for attack
can be avoided).
*: PAL/International version exclusive.
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So, that is that. This is me again. General reason why BHPL isn't added:
- A pain to get.
- More disadvantages than advantages.