I personally feel like there isn't really much of a point to attacks/abilities that deal damage split between physical and mental. For things like the normal Tail Whack, I can understand not wanting to make it as strong as a normal attack, because it's an on-command ability you can get very easily, so letting it deal damage equal to your normal attack and also adding on tease damage would make it a straight upgrade to your normal attack.
Except, Duelist's Stance for the thief adds evasion and sexiness, along with an automatic counter-attack if an opponent targets you and misses, and since it also generates threat, that means you're probably going to get a free attack while drawing attention away from your allies, and also it's a stance so you can set it and forget it to gain more damage overall. Also, Charge Weapon for the White Mage may be for a class that focuses on spells, but it still adds a bunch of damage to all of your weapon's attacks while dealing group damage when cast. Bolstering Dance is a stance, and it is constant (albeit low) regeneration to your party's resolve stat, while also giving you evasion, and Song of Courage heals your allies and gives everyone a buff to several stats, including damage. Arcane Shot is a side-grade, but because it's a side-grade it's basically an upgrade against certain foes. Frost Arrow requires a specific weapon to use, but it's otherwise literally just a stronger weapon attack. Ranger's Stance is a trade-off, but it still buffs your attacks.
If you equip the Tail Whack at-will ability, you miss out on using any of these at-will abilities, most of which are buffs or just straight up better than your normal attack, and you deal less damage. At level one, using only the starting equipment for a Warrior, and starting as a Lupine, a normal attack did 36 damage, a normal tease did 26 resolve damage, and a Tail Whack did 23 physical and 17 resolve against Ninian. Sure, the Tail Whack did more combined damage, a total of 40, but if you're fighting an enemy you're likely going to go with either dealing only physical damage or only resolve damage, because there's no benefit to dealing tease damage if you're going to beat them by reducing their hp to 0, and there's no benefit to dealing physical damage if you're going to win by reducing their resolve to 0. Thusly, the ordinary Tail Whack is overall worse than literally anything else you can do, if you have it equipped. However, I also think that buffing Tail Whack would only create a new problem, if it's better than anything else you could do. Except, that's already the case with several at-will abilities, which were mentioned before, being good, including the Frost Arrow, which is, in every situation, better than just making a normal weapon attack.
Moving away from Tail Whack, a different example of split damage is Atugia. I like Atugia, as a character, and I would love to have her in the party, especially since she has the Guarded Stance at-will power and a few abilities that do some healing to allies, meaning that I could swap Cait out for a character that tanks for the party and has minor healing capabilities (since it's basically a necessity in this game to have at least one party member be a healer in some capacity). Unfortunately, every attack she has, including her basic attack, deals split damage between physical and resolve. Her kit is practically perfect for a tank: Her Guarded Stance indicates that she's a party's tank, supposed to generate high threat so the squishier damage dealers won't take damage. Her Eternal Light is a slow-acting heal, indicating party members that do take damage should only do it rarely. Her ultimate, Shell, is a massive indicator, since it gives her a lot of health and gives her a high Threat level, basically the perfect tank ability. Her Ancestral Judge grants her a short increase to resistance and prevents her from getting stunned. Her Echoes Life ability heals everyone (sustaining them if the foe decides to ignore her) while dealing aoe damage (more threat tends to be generated from multi-target damage) and applies a debuff.
The fly in the ointment here is that my black mage turn-one fireballs, then turn-two lightning spikes, and turn-three withering bolts (and also usually at some point summons Kiyoko). Brint turn-one Cleaves (I control each action my party members take), turn-two uses Giant Reach, and turn-three Executes. Meanwhile, Atugia turn-one uses Guarded Stance, turn-two Echoes Life, and turn-three either Eternal Lights my black mage character or uses Ancestral Judge. The issue is, Atugia deals extremely low damage, and Guarded Stance takes a turn to set up. So, by the time Atugia gets to do anything, Fireball and Cleave have already generated massive amounts of threat, because they're both very high damage AOE attacks. Then, Lightning Spike deals guaranteed damage and may surge, dealing AOE damage again, while Brint's Giant Reach hits a single target like a truck. So, Echoes Life may be AOE damage, but even with the extra threat generation from Guarded Stance, because Echoes Life deals incredibly low damage (which I attribute to it being split between physical and resolve), Atugia ends the turn with the least threat of the entire team. In the third turn, Withering Bolt deals splash damage and Execute is high damage, so that's even more threat, while Ancestral Judge is, again, weak split damage which generates very little threat, even with Guarded Stance. If the combat extends further, Brint and I keep doing high damage abilities that may damage the whole group, while Atugia's basic attack also deals weak split damage, generating no threat. The only real way to make Atugia have more threat than the rest of the party, as she is now, is to purposefully make the party less efficient, but why do that when you can take Cait, who can pretty much sustain the party under a normal assault?