Elemental weakness/advantage on its own is you take about -25% damage if you're resistant and about +45% damage if you're weak.
Interesting thing is that if you look at the provisional damage formula, weak point correction is lumped together additively with the other element related bonuses. Things like Apocalypse/Phoenix/Garuda's bonuses to specific elements, Ignited Dragoon's summon effect of +fire attack/+wind resist, Agni's +fire attack buff. So the more you can stack up +element attack stuff, the less relatively impactful elemental advantage becomes for you as an attacker.
On the other hand, if you stack up +character of X element stuff (which stacks additively with assault instead) instead of +element, elemental advantage retains the same relative impact.
On the defensive side of things though, since specific +resists aren't as easily stacked up, avoiding weakness is still the name of the game since eating an extra ~45% damage for free is never fun.