Because the car is going to shift into overdrive when you are at part throttle and cruising. Then when you need to accelerate it has to down shift every time. This caused more heat and wear in the trans due to the extra shifting and also causes worse gas mileage in town as well. If you do not get past about 45mph in town then I would drive around with OD off.
I use OD through town all the time, because I usually just roll along at 30-40. It's a bit different when you have 4.10 gears though, almost no need to downshift. If I'm going be doing a lot of slowing below 30 or so and re-accelerating, I turn the OD off until I get to where I don't have to do that anymore.