There does seem to be some benefit, but not consistently, and not for all
groups. Willy Jocher, the German aquarist, found soft spring water to be
superior for spawning certain delicate South American characin fish, yet he
found rainwater was better for others. I have had problems using rainwater
on some cacti, and find that many seem to prefer our excessively hard but
alkaline water. I've noted no difference at all between rainwater and
distilled or deionized or RO water for carnivorous plants. Also, our
rainwater is quite acid and we have no lightning--the reason is that it
picks up carbon dioxide from the atmosphere as it falls, and even after a
few days standing it is still equilibrated with atmospheric carbon dioxide,
which is quite soluble in water.
Sean Barry