hi, i have a similar question. i have started collecting the water that drips from a pipe coming from my central air conditioner. previously this water fell on the ground and made a mess. it is a lot of water, about 2-3 gallons a day. i am using it to water my plants. i think it is safe for this purpose. any thoughts? anyone else doing this?
paul

it is fine. actually if the pipes are clean it is pure distilled water and drinkable. though 99.9999999% of the time you would not want to drink it due to the algae inteh pipes! but for shrubs/flowers/veggies you cna irrigate to your hearts content. both my central AC units discharge into flower beds/shrubs. and those areas are the best looking ones we have.

An evaporative cooler is very different from an a/c. While the condensed water from the a/c would be just fine for the plants, I'm not so sure about the evaporative cooler. The reason why the evaporative cooler is cycling the water, is because as you evaporate all that water cooling down the air you are left with a concentrated solution of the dissolved solids from the tap water. The reason you have to get rid of it is because it becomes extremely corrosive. Now I don't know how this would affect plants, but it is awful to steel. It is mainly mineral salts.

sorry, my response was about AC condensate, not the evap cooler.