Andy -
I think I know where you're coming from here, because I was there a while ago as well. I had read a bunch of books which claimed that there were mountains of cases of poltergeist activity - so many, in fact, that "true" poltergeist activity had specific markers, and you should expect to see them when investigating a new case to tell whether or not it was real.
The books which say that are lying. It's that simple. There is no cross section of cases to study because there are no cases. Even having read dozens of books on the paranormal, I could not tell you a single reliable case of
anything. Things like "there is usually a teenager involved; and the phenomena dies down as they get older, presumably because they don't have as many pent-up emotions" - really, that statement is just repeated a lot by people who want to believe, or because it gives whoever is saying it a tone of authority. Imagine if I were to say something that you know is a wild concept; one that you couldn't possibly accept. How do I go about trying to convince you? By making it sound like I have answers based on experience; like what I've seen gives me expertise.
Consider pixies, for instance. If I said, right now, that I believe in pixies, you would probably laugh at me because that's ridiculous. But before the mid nineteenth century, it wasn't at all odd to believe in pixies, and tons of people had stories about the mischief pixies had wrought on them.
You see what I mean? A bunch of people saying it's true doesn't make it true. I get that you are saying that poltergeists cannot have True Evidence the way people are defining it here, because of poltergeists are sentient, then they can choose to either manifest or not and you can't bottle one up and stick it in a lab.
The same is true of pixies. They could just be really, really good at evading detection.
But the fact is that people now have devoted lots of time and lots of instrumentation to trying to prove that ghosts/poltergeists/whatever exist, and in all that time with all those devices there is not one good piece of evidence that they do. It isn't that skeptics are being overly dismissive. It is that no one is delivering anything we can address.
In the end, it's a question of whether you choose to believe it or choose to not, because there is nothing else to go on. Skeptics don't usually believe without reason, so you will find when it comes to poltergeists most do not believe, and think spinning wheels about particular cases is pointless and unnecessary.
I wrote an article you might enjoy, and which might help you understand the responses you are receiving.
http://whofortedblog.com/2008/12/22/gnome-hunters-skepics-and-you-people/