Completely nullifying one hit per turn, not all damage.
I mean yes, there are battle designs that make MP Switch Move MP Up very, very good. But there's also battle designs that don't. For instance, poison pretty much nullifies MP Switch (the poison damage triggers after you complete your turn, and lowers your MP back to 0). a battle with lots of ranged, mid-damage enemies is a bad place to use MP Switch. (Examples of such damage being Choco Meteor, Math Skill, Wiznaibus, non-Charge summons, that kind of stuff).
So...really it's down to level design. Obviously if you make a boss with "this boss deals mass 999 damage once per turn", then that's bad level design. Same way it would be a mistake to make a bunch of 50 speed high damage enemies that all get stopped by Blade Grasp. Same way it would be a mistake to make a really scary battle where everything can be evaded (and thus shut down by Abandon).
(Granted, I'm just assuming that the Deep Dungeon is well-designed to not crumple to any one reaction ability. I actually haven't ever played the LFT DD--it might need to poison you more often >_>).
Quite often you only do get hit by one hit per turn (exceptions are of course Wundertwins which suffer from other factors, Bervenia, and Altimas. Excluding Tiny Ko since that's so gimmicky). *cough* Oblivions Edge *cough*. DD doesn't really crumple to any reaction abilities (Auto-Potion is the closest), but it does have it's issues with certain skill sets/support abilities being too good IMO. Though the only really necessary skill set (Math Skill) wouldn't be necessary with such a broken ability as Reanimate. Seriously, it's canceled by one move and is otherwise balls to the walls unbeatable.
Late game Choco Meteor isn't really mid-damage, it can do more then 70% damage to most of my characters. I don't think Abandon can make you reach 100% evade (though I may stand to be corrected). Wiznaibus isn't even worthy to be considered an ability, 20 damage...is just..hilarious really. Problem with say a fight in DD that inflicts mass poison on your team, is Stigma Magic. If it's the first move made (such as Adramelk's Undeath) you can just pre-arrange your units to all be hit by Stigma Magic (I don't think there's a single fight where your team starts on un-equal ground) granted if it's cast over and over and over again, it'd be a legit factor. Returning back to reaction abilities, Blade Grasp is pretty close to Auto-Potion on it's effectiveness but the one fight I really wish I had it (Altimax2, that 2 handed Altima with Gravi2 on it's attacks is terrifying) has so much magic damage going on you can't really afford to use it without sacrificing elsewhere. That is pretty much what I consider good level design, you have to give up something to gain something else. Oh and I suppose the goblin army in Nogias benefits a lot from having Blade Grasp but that's easily shut down by terrain+stop. There's also the factor that Shields in the late game are so good evasion based reaction abilities begin to wane in effectiveness. Venetian, Genji, and Paladin Shields together with lets say Feather Mantle on Ramza (bad class evasion at only 10%) will give him 90% evasion with Paladin, 100% with Venetian, and 100% with Genji. Granted I am not a fan of either the Venetian or Feather mantle (too much focus on Evasion). So let's compare a more useful Dracula Mantle with Genji and Paladin (both absolutely excellent shields). With Paladin 62% evasion, with Genji 72%. Here we see a significant drop in evasion, 62% is quite low. However, factoring in Protect I think it's quite even.
On the other hand, Ramza as most other Females will probably prefer to use Chantages instead of mantles (or any other perfume really). So let's examine it on say Orlandu, high class evasion and high hp awesome. 72% evasion with Paladin+Dracula, 82% with Genji+Dracula, 100% with Feather Mantle and either Shield. Honestly, the Feather Mantle is overkill in any of the examples, you almost never need that much evasion. Same problem with Abandon, you almost never need that much evasion on top of a shield. It has it's use of course with Glass Cannon types (such as my MA Ninja) but for them, they still suffer to magic badly. So it's easier to put some kind of damage/movement increasing ability/item. For reaction abilities, I think only Speed Save, PA Save,or Sunken State really fall under this. While for items there's the many types of Boots, Vanish Mantle, Bracer, Genji Gauntlet. All of which add more to the characters role then an evasion based Mantle (Vanish Mantle while having evasion, is more useful for the Start:Transparent, it allows an otherwise OHKO character move to the enemy he needs to move). The bonus of of using a Dracula Mantle over a Feather Mantle is also, is two-fold. It decreases magic damage (while providing decent evasion on the magic front and lackluster on the physical but also decreases physical damage), which coupled with Auto-Potion makes magical damage almost negligible. Something the Feather Mantle fails at.
This also extends to the reaction abilities problem, it's easier to take a hit and heal through it back to full (low faith+shell+auto-pot), then hope for evasion to kick in. This of course, falls apart at 999 damage abilities. But those have their own issues, in Adramelk's case (speaking about the End Adramelk and not Igros) it's only a 1-person-KO which is easy to revive through, while damaging him enough to prevent him healing 999 HP isn't too hard this late in the game. While Oblivions Edge, is both unavoidable and (I'm not quite sure here) un-mitigatable hence you are forced to either MP Switch or hope he doesn't use it. Kind of shoe-horns mitigation reactions (Auto-Potion, Damage Split) and evasion reactions (Blade Grasp, Hammedo, Abandon).
Also what Dark Holy Elf said about poison...though with my set-up I had horrible MP >.>
