I have a rule whereby if I'm riding my big bouncy bike I'll always wear armour regardless of where I'm riding. I find it removes the should I put it on or leave it in the car question. Of course most of the time I'm not on my big bike so it's not that effective.
I ride at Llandegla a fair bit and have never worn armour there; which to be honest is quite stupid. Yes it's easy, and yes it's man made so there's rarely any supprises, but for this reason (and this applies to most trail centres)it has to be ridden quickly to be entertaining. Oh and it also makes it easier to be complacent. In the case of Llandegla, if you leave the nice smooth trail some of it is really quite steep and rocky with lots of trees (obviously), I found this out the other week when I broke a frame.