I'd agree with all you've written there Mick, though I think describing falling asleep as a 'danger' is a bit of an exaggeration! There's a good chance it might happen, but it's hardly life-threatening.
I think the 'literature' on mindfulness and meditation places a little too much emphasis on the staying awake aspect (which is largely an artifact from Buddhism, where the emphasis is on waking up and become enlightened). As you rightly point out, many mindfulness students are sleep deprived, and they've been in the habit of associating lying down with sleep for their entire lives. It's no surprise that they end up in a deeply relaxed, sleep-like state, and in all likelihood, this may be just what they need.
When too much emphasis is placed on staying awake, folks often end up berating themselves unnecessarily; feeling that they've failed because they keep falling asleep.
I feel that it is far more important to be kind and gentle to oneself than to remain 'supremely alert' throughout every meditation you do. By setting up the expectation that you should be awake all the time, we risk reinforcing habits of self-chastisement; and some people even give up, assuming that they are incapable of meditating. Also, it's simply not possible to be alert all the time. The mind needs its rest, its dreamy, slow, sleepy states. I've found that great insight and creativity frequently arise out of such states, whereas a clear, alert mind doesn't allow the mind to process in that 'right-brain' way. In this respect, I've found it more useful to be aware of
all our mind states, rather than just one
ideal state. By idealising one state (awake, aware) we devalue all the others.
Finally, most students, even experienced ones, often assume that they've fallen asleep; when in fact, they are just deeply relaxed. Unless you are snoring or drooling, you are usually still awake. The mind may be in a state in which you see and remember little, or find yourself dreaming, or in some kind of hypnotic or trance-like state; but the fact is, when the meditation ends, most people immediately come out of it. When they are asleep, they don't.