First off, let me say that I have knowingly recommended these tyres because I do feel that they offer satisfying durability and performance. In fact, I've just recently purchased a second set, because I was happy with what the first pair brought me.
With that said however, let me relate what I think killed my first pair:
The tyres still have plenty of tread and studs left, the issue is that the casing is worn on the inside where the studs push through, and so inner tubes rub against the studs and develop leaks. I know that I ran these tyres above the recommended pressures during my last winter with them, and I'm thinking that this is what eventually destroyed them.
What I came here to ask is whether some of you think that this is in fact what must have destroyed the tyres, and if I can get some opinion as to whether I might get longer life out of my second set by running lower pressures?
It's not so much that I couldn't accept getting no more than 3 or 4 good years out of these tyres, but I'd really like to hear from someone else who has heard of the casing of studded tyres wearing out.
Update (18/12/2012): Well, I contacted Schwalbe and was informed by their customer support that the Marathon Winters were improved with wider-based studs in 2011. Hence, I could hope for better durability out of my new pair vs. my old pair purchased in 2008. The only caveat is knowing whether or not the tires I received from Wiggle were indeed manufactured in 2012 (or post- design change, in any case). I suppose I could take a pair of pliers to the tires, rip out a couple of studs and compare them... However, given that I'm not going to return them regardless, I may just live in doubt and keep an eye open for signs of wear on the inside when I have reason to remove them.