Now that it's spawning in every single dungeon, let's talk about it.
Personally, I dislike it. It's great if you are playing a "gold cheeze" build (Fabulous Treasure + Transmutation Seal, for example) which allows you to level up in there while creating tons of popcorn on the main floor, but more or less useless if you aren't. Under normal circumstances, why would I ever want to waste gold on spawning an equal level monster? Perhaps if I found both WONAFYT and a Balanced Dagger this could be an interesting prospect, but I don't think it's worth it outside of these special circumstances. I 90% of the time just see it as wasted potential.
My thoughts are:
1. It's too expensive to spawn one equal level monster for 5g in most runs
2. +1 level enemy would be more useful
While we're talking about subdungeons.. Venture Cave (and possibly others) seems to be spawning 3 of them. Can't remember if that's normal.