Should I MLB my imaginery element

Personally I prefer having multiple to get the whole team's nps up faster rather than just one servant. I'd rather wait an extra turn or two to get 3 np's all at once rather than have one earlier. Usually I try to wait until I have 6 or 7 copies of a CE before mlb so I can still use the others on at least my starting servants. But that's just me. I'd say it really comes down to personal preference as well as what other servants you have (like a servant that can give the team 20% charge). Sometime I'll use both merlin and Ozy for a combined 40% charge so I don't need the mlb.

With it limit broken, it lets you use Arash's NP first turn with his third skill at level 6. It's very useful if you only have one copy of kaleidoscope.

I would limit break them, my farming team has one MLB Imaginary Element and the rest of my team has stuff like bond or exp gain.

There are also plenty of event CEs that get you to 50% NP, and if you take a maxed out Waver support, you can get to 100% very easily. So I don't value the 60% very much. But an extra 15% makes a lot of difference for Servants with medium NP batteries like Arash, Nursery Rhyme, or Shiki Assassin.

Personally, I used to use a non-MLB Imaginary Element a lot (only had 1 and no Kaleidoscope), but nowadays I value the CEs that have 30-50% charge plus some other buff. So my IE usually sits in the backline.

Then again, I don't put a lot of weight on 3 turn clears. I don't mind building NP charge on wave 1, then 2 turn clear the rest.

So, if I had the choice, I'd probably MLB it, as it would then be much more useful in certain situations.