Most cyclists I see on the street can stop at the red light staying on the seat with one foot on the ground, while I was busying with dismounting and mounting (this is Manhattan with a red light every few blocks).
No, actually if done properly one can stop and start quite quickly without staying in the saddle. In fact many who stay in the saddle start up awkwardly. You just shift to a good gear as you slow down, put one foot down and then rotate the other one so that the crank is pointing forward. When the light changes you just step down on that pedal, which puts you up in the saddle and propels you forward at the same time.