Depends on the region, too. If it's flat where you live, it's less of a worry. In places that get wet snow, or get snow on warm roads that freezes immediately (I'm thinking of the northwest here), traction will be much more of an issue than somewhere that's really cold.
And summer tires get hard as a rock even *without* snow, so you shouldn't even be considering taking summer tires to the snow. In most areas, all seasons are okay, but I'd recommend a proper set of winter tires for a car like a BMW, so that you can have GOOD summer times for the summer -- all seasons are fairly bad at everything. I was once unable to back my Z3MC up a hill, in 40 degree weather, because my PS3s were too hard to get any grip. Totally dry.
That said, had my GF's Subaru up on the hill this weekend, in sub-zero temperatures, over 8k feet, on all seasons. Continental DWS, to be specific
Only reason I've had my eyes out for the F31 was to get an xDrive touring w/6MT which doesn't look like it'll ever happen... but if it did, you can bet I'd have proper winter tires and proper summer tires. I am simply not interested in ever putting chains on any car I own. (when it snows here, in certain conditions, you are required to run chains unless you have AWD.. such as this weekend).