Since this was posted in the SW
forum I have to guess you want to use it in the SW
tank. Since salt mix has various minterals and trace elements within it already adding rainwater that could include other various minerals including maybe some that the salt mix has could cause a chemical inbalance in your tank. This inbalance could lead to excess algae growth or quite possibly unknown wilting of inverts or invert dealths.
While the local enviorment will play a big role in whats in the water just like it plays a big role in the quality of well water. If your in a fairly rural area then your rainwater will probably less contaminated than if your in an industeral area or if your down wind of an industural area. (acid rain)
The best water for SW
tanks because of the potental for chemical inbalance is RO
water. Since this water is for all intensive purposes pure its free of extra minerals and thus is easier to acheve the chemical balance that the salt mixs are designed to give us.
Rain water is probably desirable for plants for the same reason that RO
water would be undesirable for plants. Im not a plant person so I dont know if this is indeed the case but I can see problems comming from giving house plants pure water vs tap or rain water.