Dot crit is not reduced. Dots hit one target.
I think you can make a case for changing mage rain crit, without bringing in a different mechanic and saying its the same.
There are no other mechanics like rain that I know of, but there are a few that are similair.
1) Pbaoe (blue gem)-> 1 spell, 1 mana cost -> as many targets that you have aggro on ( I do not know how high this will go never tested but I know it works up to at least 8) That is 2 more than a rain.
2) Targeted AoE (green gem) -> 1 spell, 1 mana cost -> up to 4 targets iirc
3 DoT's -> 1 spell, 1 mana cost -> Lasts the duration of the DoT or until mob is dead and have long enough duration that multiple dots can be stacked on a mob by 1 dotter. It is show below that not only will they have enough time to cast the 2 dots and keep them up but they can also do other things while having them up such as nuke heal etc.
Necro dots-> Spell: Relic: Marlow's Cremation -> duration 36s , casting time 5.5s
Spell: Archaic: Claws of the Chill ->duration 24s, casting time 2.75s
Shaman dots -> Spell: Relic: Scourge of Life -> duration 24s, casting time 3.5s
Spell: Plaguewind -> duration 54s, casting time 7.75s
Druid Dots -> Spell: Relic: Doomswarm -> duration 54s, casting time 5.2s
Spell: Crawling Swarm -> duration 54s, casting time 5.3s
The illustration I am trying to make is that multiple DoT's can be kept up on multiple targets at full efficiency (not falling) and the caster still has time to do other things such as nuke or heal. With mage/wizard rains when using multiple rains you can only keep 1 up at a time as the first one ends or the last wave hits as the next one lands. In between rains there is minimal or no time to nuke, cast mod rods, pet heal, clickies, etc.
Again I am not saying that these are the same, I am saying that they are similair or close.
But again, if by getting crit rate increased on Rains comes at the cost of decreased raw damage I am 100% against it.