You are right- flap positions 1-15 will not increase very much the drag.Selecting flaps during descent,especialy on classics,it's not so indicated due to posibility of an increase in speed (gusts,tail wind,etc) and a low flap limit -230 kt.
with thrust at idle,speed 210 kts (min clean) you'll descent normally with around 1100'/min. So ,if you descent with 1500' and you start extending flap,there is a big chance of overspeeding the flap(due to external factors like wind ).You'll have to use speedbrakes,and this is not such a nice manouvre.And if you have a tail wind of more than 20 kts,then you have some work on your hands.
The 737 has good aerodinamics and poor speedbrakes,it's sometimes hard to 'brake' and reduce speed.
That is why it's indicated to start extending flaps in level flight.
Brgds Alex