Not really a problem leaving modern winter tires on all year unles you do performance driving (winter tires are rated to 210 kph in Germany). Anyway, after I broke my foot I left my Dunlop M2s on the X for one summer. They wore quicker than summer tires but not too badly. I spotted a couple of SUVs at my office using winter tires year round. We are living 38c/100F days for a number of weeks every summer and unless you are doing donuts in the car park you should be fine. Ofcourse summer tires will perform more adequately in the summer if you are driving more aggressively.

Also, I have had Michelin X-ice on my pickup truck for 30.000 miles now and I leave them on year round (as the grip on the rear is awful I need the extra traction). There is probably 5-8000 more left in'em. I am very impressed because I expected them to wear out in 15 or 20000. However, the performance of a Tundra V8 is not quite comparable to an X5....

If you get 70k miles out of a set of tires they are rock hard and extremely dangerous in the wet. I get rental cars with rocks on them like that all the time. No grip whatsoever and disgustingly dangerouns IMO. There should be a law against such tires.