You are forgetting something very important, and that is that talent points are precious. And those other skills do other things.
Constitution increases HP, it ALSO increases your stamina and reduces the cost of noncombat actions
Perception might be "only" 2% def instead of 2.5% def per rank as you say... but unlike armor it has more going for it. it gives you items, and boosts your damage via crits, and land debuffs.
Again, I am mostly talking about Tia, but there is an argument for increasing armor for MC as well. Perception is 2% per 2 levels, so 1% per level, thus it is much, much less effective for tanking. And increasing energy for Tia is not high priority, it's not that you can easily run out of it.
Oh, you were talking only about Tia and not the MC and Lyvia.
Tia specifically has fewer things competing for her talent points than the MC...
However, tia is not a tank. Tia is DPS.
Tia isn't a tank, she has poor defense and high offense. Tia is a DPS. Tia has the highest damage in the party. Her damage is higher and her defense is lower than Lyvia.
She has a completely useless taunt skill, this does not make her a tank.
Not only wastes actions (by taunting instead of killing the enemy), but also even if you go full tilt into boosting her defense she will still get annihilate by enemies if she tries to use it.
Also, she lacks the agility to cast the taunt skill before the enemies attack and damage the rest of the party. To be an effective tank one needs massive agility so that they can cast their taunt skill before the enemy attacks. Massive HP and massive defense so they can take hits for days.
Tia cannot do any of those things.
Far better to pour in her talent points into strength. Where she gets a huge boost to damage.
Also Tia is very useful in harvesting resources on the map which benefits from a bunch of stats which compete for her talent points
Tia is most definitely a tank and she is a tank precisely because of taunting skill which is by far the most valuable battle skill over all party members. Taunting concentrates all agro on Tia (this is a definition of a tanking skill) AND it provides an additional bonus for defence. Thus Tia is a textbook tank even if her starting defence is marginally worse than that of Livia (I am not sure about the last fact, but I will take your word for it).
If you spend Tia's skillpoints wisely (that is mostly on armor and some on constitution and even less on perception and strength) Tia can sink damage by any enemy even on reasonably low levels. And of course with taunting skill she does that not only for herself, but also for MC and Livia.
With Tia as tank and no Livia (she wasn't avaliable yet) I was able to completely clear all cultist basement including the prophet in one go without using any potions on level 15 or so. Of course this becomes even more relevant on higher levels.
Thus you can make Tia take care of defence for the whole party and use MC's and Livia's skillpoints to increase attack of the party and do other valuable stuff.
