All seasons are not all seasons. And M&S ratings aren't effective in snow.
The truth of tires is that winter tires work better in anything below 7*C. Anything lower than that and the rubber compounds in non-winter tires get hard. Some A/T tires perform better than others in the winter, but none of them will perform as good as a true winter tire, and anyone who says otherwise has never driven on real winter tires.
The Duratracs are kinda polarizing. Some people love them and they last forever, some people they're trashed in 60K... Either way, my dad switched to them and the next year bought a set of real winter tires. Needless to say, he switches them seasonally. So even though they're pretty good for a mud tire, they're still not a winter tire. If I absolutely had to run only one set of tires, they'd probably be it. Also, being a digital tread, they're nice and quiet on the road.
That said, I run Hercules Avalanche Extremes year round. They are a fantastic winter tire. I have about 30k on mine and still have 50% tread life left. I have run them all year round for 2 years, driving my truck like I stole it. I keep my pressures set properly all the time and they're wearing great. If I switched them off during the summer, I'd easily get 4-5 years out of them. I may replace them this summer with an A/T tire and keep them for winter tires, but that's still up in the air.
I've got a lot of specialized tire experience, so I'm a bit of a stickler for proper tires. Winters are winters, all seasons are for the other 3 seasons of the year. Avalanches are the only winter tires I've seen that won't wear down to nothing in the summer, and I would never run a non-winter tire in the winter.