Posted: 17/03/2014 at 15:07
Water butts may overfill and thus overflow, but I've never heard of them causing rotting to decking - the water will just drain away. We have several butts around the place, some of them there for many years, and all works fine. They are not more trouble than they are worth once you get used to them, because your plants will love the contents so much more than tap water.
I have a jam jar in one of the unused holes in one of mine - from which all the birds like to drink - no matter how many water feeders there are about the garden! I guess they are there for different types of fittings - I've never thought about it, but off hand cannot think of another purpose for them.
You can stretch some old socks or tights over the holes to let air in and out but keep the insects and/or mozzies out.
We have a waterer diverter, and two butts linked together so that when one fills the other takes over - they're full now but will be essential later on - I hope.