I've always been converting this glyph immediately. Revealing three random tiles not only isn't very helpful, it actually removes future healing potential. I can get the same healing effect from just revealing three tiles that are actually near me. So why would I ever want to take up one of my glyph slots with LEMMISI?
It's basically meant more as a primitive healing factor, which is why there's actually a bonus point reward for using LEMMISI repeatedly in one stage. Don't use it to rob your future healing potential, use it as your healing potential.
If you're not a Wizard or Berserker, you make even with the mana cost. Wizards will regain 1 Mana, while Berserkers will spend 2 extra Mana. It's primarily for usage when you don't want to perform exploration directly. Some reasons you might want to do so include:
As a primer to actual exploration. You have some damage, want to heal it before advancing. Use LEMMISI to possibly see what might be a better direction that you want to head in, rather than bruting through the darkness towards what may have been the worse direction to go.
As an extension to the above, use it while facing off multiple visible opponents at once. This lets you get the normal healing effect of exploration while exploring the more remote parts of the map, instead of what is right near you.
If you're in a situation where you can't directly explore due to big tough fellows in the way, but you can defeat one opponent if you had the aid of exploration regeneration (either just to get back to full health, or because you can beat the enemy's regeneration rate), then this helps.
You've run out of normally reachable exploration, and there's some blackness that's buried inside of a bunch of stone blocks. Unless you're a transmuter (in which case you don't want LEMMISI in the first place), it's far cheaper to use LEMMISI than to dig through the walls.
If you're a Sorceror, then you get extra healing on account of your ability. Since the mana cost equals the amount you'd regain from actual exploration, it's actually better healing.