Right. It's been mentioned by previous posters, but never explicitly stated: the name of the game is stacking weapon bonuses of Assault weapons (generally - depending on your eidolon and if it gives you HP you might consider throwing a Defender weapon or 2 in there).

As NO ONE has the time/resources to level all 6 elements weapons grids concurrently, many people pick a main element, and perhaps a secondary one that can help offset it's weaknesses if your main isn't Light or Dark.

I.e., I main Fire, which means i suck against water events/raids, so my secondary team is Thunder.

Now, that DOESN'T mean I don't have 6 teams already assigned, one for each element, because I do. But I don't waste my time upgrading the dark Assault weapons I have - all my rares go towards enhancing my Fire weapon board. Even without any assault weapons leveled beyond 3 or 4, I still have no trouble clearing events with that element if I need to, as you can gain a TON of ATK/HP from your eidolons. For example, even though I main Fire, I still actually use my Wind team for this Yggdrasil event - it's good enough to kill the Expert raids based just on my Eidolons. Of course, that's mainly because my Fire grid isn't complete yet.

Also, just to be clear, I definitely save SSRs and MBL what I can, because you can't predict what you'll get from the Gacha. While I don't farm any of the Disaster raids except my elements, I also keep the assault weapons that they drop. You never know what you might want to do in the future!

Just some food for thought.