It seems ludicrous to me, but it is actually illegal in some states to collect rain water in this manner… If you were somehow harvesting tens of thousand gallons of water with some industrial sized system, I might understand, but when we are only talking 100s of gallons of rain water in most cases it is crazy. Anyway you might want to check your local authority if you are concerned about the legality of this where you live.
If it is legal to harvest rain water where you live, this is definitely something you should consider doing. The rain water can be used to water your garden, and in an emergency you can even drink it if you treat/filter it first. This rain water collection system is easy to build and very effective. As it is stacked vertically it is also great for people with limited space.