Honestly, the way i see it (and the way i always did my vids), you have to do what works best for you. cause it's not really wrong to think both ways.
Not everyone can have 6 elements to function with, but on the other hand, not everyone can run with just 1 element.
plus, like it was said, it still depends on the tools. I'm still hesitant to beef up my water and thunder teams cause the selection of kamihime I got isn't too good (especially in the case of water).
but on the other hand, elemental advantage is still roughly 50% more damage... including debuffs lowering defense and buffs increasing attack, that can add up to a lot. case in point is my fire team vs my light team. fire with elemental advantage is doing just as much as my light team without including any buffs and debuffs if my fire team has the advantage, but my light doesn't. and there's roughly a 50% difference in assault and maybe a 10% difference in elemental attack that my light team has over my fire one.
not to mention, you take less damage too. light is my best team, but barely survives crom GO cause you guessed it, no elemental advantage. So the trigger ends up killing off some of my team and it just gets worse from there the longer the fight takes. and this is with a Managarmr as a friend eidolon and a skill lv20 herc weapon (totaling 170% light elemental atk with my own eidolon). offense isn't always the best defense... i'm literally seeing a team of about 14k HP on average get it slowly nuked away until the rage mode hits and the trigger puts some girls out of their misery cause my only real defense is atk debuffs, sol's healing, and random tiamat summons.
This game is extremely RNG heavy, so causal or hardcore, you still have to play with what's thrown your way.
i still would get everything you can from events though, cause it's still free resources just waiting for those willing to grind them. And say you don't need an SSR weapon, it's still fodder for skill leveling which to be honest, is the best leveling resource in the game considering skill lv2 SSR guarantees success on anything, even a skill lv29 FLB weapon. And in the case of SSR eidolons (and the two noteworthy SR ones), even if you couldn't care less about the scenes, it's still 100 free magic jewels to do whatever with. Free/cheaper players need this more than anyone since they won't be buying anything anytime soon if at all.
one thing is certain, it's a problem if you can't clear the content. but the way I see it, how you clear the content is where the difference lies. If you can clear with a team rocking 25k HP, have at it. If you can clear with a team using 2 phantom lilims, then go for it (someone on DMM did something close to this in a rank 6 AQ vid in fact). It's ultimately up to you on how to build your team. All anyone else can do is just give suggestions on what might be the best options.
at the end of the day, you still have to play your own account. finding out what works and what doesn't is the key, cause this game has it's ways of shutting down any strategy. for all we know, there could be a boss trigger later on that instant wipes your front 5 girls if they do too much damage... learning how to adjust instead of sticking to one strategy is more helpful than you'd think.