half of this i agree with, vulnerability while meditating will cause it to become useless in progression where it really matters, because the main viable points to use it would be between phases, at which point there is usually large damaging moves incoming, phase 3 > 4 transition in t13 is the perfect example.
with that said, as to the post i've quoted, being "in combat" wont mean "i can hit the boss so i should do that instead", it would simply mean the boss as enmity from the party (because it has been pulled), phase transitions often render bosses invulnerable due to various reasons as we all know, this would be the time to use meditation.