So, I did some thinking on the two defensive limit breaks (healing and tank), and have considered ways they might be fixed.
Healer's issue, as I mentioned before, is that it only solves a minor problem that could probably be solved with a medica/succor or two. It does what'd take you probably 6 seconds of time, whereas the damage ones can shave off minutes of time. It also does nothing to aid with problems down the road (likely coming if the enemy got you that low in the first place). So, my proposal for the healing limit is, on top of the existing effects, add a regen to HP/MP/TP when used, possibly based on weapon level of the party (Like the damage limits are). Duration should last a decent amount of time, at least 30 seconds, possibly up to a minute. This not only handles party healing so the healer can focus on the tank and/or damage the boss, it also reflects the support nature of the jobs that get it by allowing the party to fight at their best.
Now, tank's issue is simple: The duration is too short. While negating up to 50% damage sounds nice, it's only really usable if you know a big attack is coming and have no way to circumvent it normally. I know of only one attack in the game like that (Ultima hard mode's final explosives). If the duration was noticeably increased (say, 30 seconds to a minute), it would be usable to help a party recover or attempt things that would normally be too risky.