Tyre manufacturers say if you can benefit from M&S tyres, and can only have one set, then it's best to have them year round. That's exactly what I'd do, if I didn't have the luxury of 2 sets.
Some winter tyres wear very fast I gather, but not all... there's only a small difference in wear between my snowproxes and my ventuses- pretty equivalent tyres on a heavy car driven by a lead-footed idiot. Lets say, oh, 10%, handwavily. My old icebears wore just a little faster, but still not terribly so (cheaper tyres, I suspect they were just not as well made tbh)
The grip difference does exist, but, it's at its biggest under perfect driving conditions where a difference in grip is least troublesome. As driving conditions get worse, the tyres get better, I'm happy with that- if you want to think of it as "average performance", the benefit outweighs the loss by a big margin. I get an increase in noise and a marginal drop in economy though...
But yep, I reckon it is still better to have 2 sets of wheels (and remember, though it's an upfront cost, if you sell the car you can also sell the wheels and recover the investment). Probably more significant is that winter tyres work best when new, all those clever sipes etc fade faster than the rest of the tyre- doesn't make them useless but does make them past their best. None of this is gamechanging but still worthwhile IMO.
From a mountain biker's point of view, when I did a summer on the icebears I loved the extra offroad grip (when I swapped cars and went with 2 sets of wheels, I forgot all about it and instantly got stuck in a ditch!). Muddy dirt roads, race parking in fields and everyday parking in verges and the like can all be handy. Towed a freelander out of a very muddy pitmedden once, with my focus, that was worth the price of admission alone!