Batty - for what its worth, we leave a valve type cup of water with our 2.5 year old DD1. We would have to do controlled crying with her if we took it away. Its a recent thing since she was ill and wanted water in her room (which seemed like a perfectly reasonable request to me!). She hasn't always had it, but she does tend to be a bit OCD about stuff in her cot, and we tried to take it away but the hysterics just weren't worth it.
I never even considered that there might be a choking risk - I can't see how its much more of a risk than letting her drink in a different room to me in the house really, as she would need to sit up and be fully awake to drink (don't think she does actually drink it though, but that's not the issue).
I'm not sure I would leave one with a 1 year old though, they just aren't as co-ordinated or sensible? But that's for the parent to decide.
Virtually everything we do carries some form of risk, and parenting is just about juggling the relative risks and rewards. Some will not want to take any risk, some won't mind. Since the risk seems to be miniscule, I think there are more serious issues for everyone to get het up about.
So I think yes, given the arguments presented above, YABU to think that the risk is a myth. However, YANBU to consider the risk small enough to take.
For those that wouldn't leave a non-spill cup of water with a toddler - it would be interesting to know at what age you would consider leaving water in their room to be 'safe', and why?