There are plenty of cable locks on the market, both combo and key. I used to use a cable lock for very quick stops but found, surprisingly, that it took me longer to use than a u-lock because the cable lock I have has to be coiled a certain way for storage, otherwise it's a mess. I also found that the numbers on the combo faded to become almost indecipherable and the combo lock got rather finicky: even in the right combination it took the perfect alignment up to a millimeter accuracy to open it, and even then it would give me trouble sometimes. I probably looked like a thief at times, cursing, realigning and pulling with a lot of force.

A U-lock on the other hand is rigid and requires no wounding/unwounding, and if you treat the keyhole right, it should be pretty fast to lock/unlock it. It also does offer better protection than most cables if used right. And it doesn't have to be heavy - there are some cheap light U-locks around.

But that's just my experience with one cable lock. Others on here use cable locks for quick stops and like it.