Over at Marginal Revolution, an emailer and former cab driver reveals a fascinating reason:

When I first started driving a cab, I drove the shortest route – always, I’m ethical – but people would accuse me of taking the long way because it wasn’t the way they drove. So, I learned to go their way ending up with a lot less grief and a lot more money. If you’ve ever wondered why a seeming professional cab driver will ask you how to get to your destination, this is why. Going your way means they’ll make more money and they won’t be accused of ripping you off.