I put it on Fritillaria, and would instantly do so again if I got any more rainbow skill blooms.
Simple reason: her ATK goes up by 2% every time a special ability is used, and increased skill comes with an increased chance for the skill to trigger, meaning her combat efficiency goes up slightly with more than 2%.

Alpinia (which i am actually using now) or any girl who have same ability. Life leech makes them survive literally 2X more if they use enough skills.

Thanks people!