While you can technically leave a ball valve partially open for extended periods of time, it can have negative effects on the valve overall. For instance, a high-pressure drop across the ball may lead to a damaged soft valve seat.
While you may use a valve with a ball for moderate throttling, these types of valves are best suited for basic functions like starting and stopping flow. If you know how a ball valve works, you can use it efficiently for a wide range of functions by placing the ball valve handle in the appropriate position (only giving it a quarter turn, for example).
However, the fact remains that it's unwise to leave this type of pivoting ball valve partially oven for an extended period. They serve best as a type of gate valve where you need to control when the flow starts and stops.