Woops. Well, I knew what I meant in my head. If you, reading from somewhere else in the internet, cannot tell what is in my head, then... poo on you!
Turquoise Nymph's "niche" is only as small as Quintessence's. They serve the same purpose and require exactly the same type(s) of quanta. Nymph goes in a deck where you think you might want more than a couple Quints.
It's a little slimmer than Quints, for a similar reason as Poseidon vs. EQ.
Nymphs require a large amount of quanta, as such, you need to generate a fairly large amount of quanta in order to fuel it, compared to quints. Also, most people have at most 1 or 2, and if your strategy requires you to quint a creature no matter what, it's safer to go with a large amount of quints rather than 1 or 2 nymphs.
This means that the nymph's current use is like Poseidon: Great to have just in case you might need it in like a rainbow deck, because it's better to have one than not, but hard to build a strategy around it specifically.