Energy is pretty cheap in space. You also don't need a whole lot of power as even quite low thrusts will eventually get you where you want to go.
It's not as simple as going from point A to point B.
'Points of interest' in space (like asteroids) are not stationery, but flying as incredible speed.
For example, when NASA sent a spacecraft to study Pluto, it took 9 years to get there, but they only had 1 day to actually study Pluto.
Only 1 day because the spacecraft couldn't just 'stop' once it arrived.
Relative speed during Pluto flyby was 13.78 km/s (49,600 km/h; 30,800 mph) so it would requite extraordinary amount of energy to 'stop'.