I have 2 of these now and I'm really struggling to find a good use for them. Luciferase seems to be better in almost every situation.
Lobo hitting your light-generating creatures? It can also neuter your nymph! But it can't hit luciferase!
Charging up a fat Hope shield? Hope and light nymph are both extremely expensive. Using either card delays the use of the other tremendously, and rushes will kill you. With luciferase, you can quickly and cheaply turn all your dragons (or whatever) into Hope boosters right after playing your shield.
Healing? Here the light nymph is kinda useful for sustained healing, but luciferases and holy lights can heal you much faster for much less quanta. This appears to be the only situation where light nymph has some use, but it faces heavy competition from: Heal, Sanctuary, SoG, Amber Nymph, Druid Staff, etc etc. Every single other option is either way cheaper/faster, or in the case of Amber Nymph, more powerful of an effect.