More Heidelberg/Karlsruhe, but here the roads are kept so clean in summer that there is no need to wash the summer tyres .... I have just got back from Tübingen, and in the short space of half an hour I counted at least 7 fallen leaves that had not been swept up.
The rule of thumb for changing to winter tyres is from O to O - from October to Ostern (Easter) - when the temp doesn't get above 7°C. It's not illegal to drive all year on summer tyres, but if you risk or are involved in an accident in winter weather you can be fined for contributory negligence (€60 to €120). The govt didn't make winter typres compulsory as there are those who simply don't drive if it's snowy, and it would be unfair to force them into the cost of purchasing and changing tyres.
There is no doubt though that the grip is much better in winter.
I've gone for a compromise on my new car, against the advice of my garage, and gone for all weather tyres. Where we live you are unlikely to get much snow if any, and for virtually all winter conditions we are likely to experience all weather will do. In really bad conditions I simply wouldn't drive. Somewhere south like Freiburg where you always get lots of snow, winter tyres would be better. Small loss of fuel economy, and in summer in hot weather and at high speed on the Autobahn the stopping distance, whilst adequate, is longer than summer tyres.