Reviving this old threat, just because it is useful for new players and maybe some others as well.
The main fault of RO2 is that some stats grant a static bonus, whereas others grant a fractional one. That's just a ridiculous stupid concept.
STR: +22 damage per 11 points.
VIT: +66 HP per 11 points
These are flat boni. You always get +20 damage, no matter how much damage your weapon deals.
+22 damage is 10% when your total attack power is 220, but it's 1% when your attack power reaches 2200.
AGI: +44 Crit. Bonus per 11 points.
Now, this also looks like a flat bonus, but it isn't. As soon as you reach level 50, it takes approx. 44 points in Crit. value to get +1% crit. chance (aka. crit rate).
This crit. rate isn't affected by the damage of your weapon. +1% chance to deal double damage equals +1% overall damage.
(99x normal damage + 1x double damage = 101x damage / 100 hits = 101% damage = +1% damage bonus)
That's why I did the above calculation with +11 in a certain stat. Since +11 AGI always grants a +1% damage boost,.
If you have an attack power of 2200, +22 damage due to +11 STR also equals +1% damage boost.
Attack Power < 2200 => STR better than AGI
Attack Power > 2200 => AGI better than STR
Now let's look at Battle Tactics:
It grants an +0.4% crit. damage per INT.
+10 INT = +4% Crit. damage
The overall damage bonus is calculated this way:
% damage bonus on crit * crit. chance = overall damage bonus
+11 AGI = +1% overall damage,
+11 INT = +4.4% damage on crit.
Therefore you need a crit. chance of 22.73% to be as effective as AGI (or STR with 2.2k attack power).
To sum up the obvious:
Since STR is better than AGI, as long as your attack power is less then 2200 and you need a crit. chance of >23% for INT being betther than AGI, leveling INT is even worse even if you have Battle Tactics.
To finally answer your question:
Take the INT stat you have and the crit. chance and calculate the damage bonus in the way I have shown above.
Then compare the damage bonus with other skills (f.e. Concentration or Berserk) and you get the answer whether or not it's wise to level it.
My general answer is NO, unless you get gear that gives a significant bonus to AGI and crit. chance.