Low priority in comparison to other damage mitigation skills like Slayer Defense, Hardy Physique, and Deadly/Fatal Resist.
There's math that goes into this answer, revolving around the concept of "effective HP" (eHP). Your eHP is the inverse of damage taken. If you have a skill that halves damage (1/2), your eHP is doubled (2/1), meaning you can survive double the punishment. Day/Night-Attuned is this, except it's only active half the time (1 - 1/2*1/2 = 3/4 damage = 4/3 = 1.33x eHP).
Slayer Defense reduces damage from Slays, which is a little difficult to estimate the impact of. The damage multiplier for Slay is 1+S*(100-D), where S is the number of types slain and D is the skill level of Slayer Defense. Without the skill, a unit takes double damage from 1 slay, triple from 2, and so on. With Slayer Defense 20, that becomes 1.8x from 1, 2.6x from 2, and so on. This means eHP increases more when more types are slain. Slayer Defense 50 halves the effectiveness of Slays and has the same effect as Attune with 1 slay, but double the effect with 2 and so on, scaling linearly. It's safe to assume that your Guarders will very often face at least 1 slay, and endgame often has them facing 3 or more. Slayer Defense 35 against 2 slays (2.3x/3x = 0.77x damage taken) is roughly equal to Attune, as a point of reference.
Hardy Physique scales exactly the opposite to Helmet Split. The more of either skill you have, the better returns you get for stacking towards the cap. Hardy Physique 50 = 1/2 damage = 2x eHP. Hardy Physique 60 = 2/5 damage = 5/2 eHP = 2.5x eHP. The max Hardy Physique 80 = 1/5 damage = 5x eHP, which makes it much more effective than Attune.
Deadly Resist allows units with Parry to negate damage. This trumps all other damage mitigation against units without Dimension Slash and is the main reason why "Impregnable Mercenary" (Deadly Resist 60, Parry 60) is such an amazing title for Noel, since it gives her a natural 100% Parry chance and reduces enemy crit chance by 60%. Fatal Resist works the same as Hardy Physique and scales similarly, but only for Lethal Critical. It's important to note that Lethal Critical actually reduces Defense by a further 25% when the unit crits, so reducing that to zero makes it more impactful than Hardy Physique if enemy crit chance is high.