Yeah, this is one that can drive your average ebayer completely bonkers. I have to admit it used to bug the life out of me when I first started, but now I don't even put any potential in watchers on a BUY IT NOW option.
As has been said here, people hit watch for a variety of reasons. I'll watch items that I have a personal interest in for research value alone, absolutely no intention of buying it.
Of course there are those that are mildly interested, not quite ready to take a jump and grab it, those that are seriously interested, but are going to shop around first and score themselves the best possible deal. Even those that put it on a watch list to show Cousin Ethel because it's one just like she used to have, and so on.
Waste of time trying to second guess them, just make your listing as effective and enticing as possible, that's the absolute best option :)