Dutch & German ATC often give you min rates of climb/descent to maintain so that's another reason.
I have heard a rumour that certain unscrupulous individuals have also been known to operate in V/S mode in climb or descent because they want to sneak above 250 kt below 10,000 ft, without getting hassle from ATC.
Because the speed ATC "see" is the one that's selected, and because V/S mode has priority over selected speed mode, if you select 250 kts on the speed but also select a suitable rate of climb on the V/S, and manually select climb thrust, it will accelerate below 10,000', still with 250 kt selected.
It's even easier in descent, because you don't have to manually set the thrust, as long as you have selected a decent ROD ( >2000 FPM), it will maintain the high speed right up to alt capture and once again all that ATC see, apparently, is the selected 250.
Sounds kind of b*****ks to me, but that's the way it's been explained to me by a couple of guys I've flown with anyway.