In reply to Timmd:
I've had a go at this! My trail runners are really stable, as I mentioned before I believe that's due to their really low rise and flared rear sole. It keeps your actual foot sole closer to the ground that boots do. It is actually quite hard to get them to go past the 'tipping point' when trying to simulate an ankle roll. Of course once they go there's no support.
The Quests are also wide at the back, and due to the high lacing it's actually really difficult for me to roll my ankle far enough to cause serious injury (caveat: I know this won't be the same when you've battering downhill with a heavy pack on).
The winter boots are, obviously, a step up again.
When I've tried on any of these mid height boots they feel a bit like walking in high heels (I imagine...). They have the looks of mountain boots, but they usually don't have locking eyelets that are very 'deep' around the side of the shoe, and as they're mid-height the lacing doesn't go high enough to offer any real support. Couple that with the Zodiacs and Mtn trainers having stiffish soles and I just thought they might be a bit more likely to be unstable.
In saying that there's been some good words of encouragement for these types of boots on the thread. Should maybe trust that manufacturers have tested and thought about this before releasing a product, when do they ever get it wrong