your chain can rust up pretty quickley after rain but I just dry my bike off straight away, it dosent take long and I have a spray called anti rust that repels the water but unlike wd40 it lubricates it really well .i do that every time it rains and once a week regardless ( just simple maintenance )
If you use something like Finishline then you won't have any such issues. I've never dried a chain after riding and never had a chain rust on me.
to the OP, I know what you mean. Logic says that it shoudln't be a problem, and despite riding my Felt in all weather over winter (except snow) I was a bit upset when the skies opened today whilst it was parked up outside my office.
At the end of the day, it's not made of sugar. Just keep on top of keeping the grit off moving parts like the transmission, bottom bracket and headset, use some good lube and give it a proper clean every now and then and it'll be fine.