Anything that uses a rotor to take-off vertically is most inefficent when lifting, hovering or moving slowly. This is most obvious when a helicopter is at max weight or in "hot and heavy" conditions when it might not be possible to take-off vertically.
In order to lift vertically, thrust must be greater than weight. This is not true for a "conventional" aircraft.
If you use a rotor only for lifting off, then that rotor becomes nothing but extra weight (and complexity) that you don't need. Maybe you could use the stopped rotor to generate some lift in a similar fashion to a conventional wing but as others have pointed out, to make a rotor blade which is rigid enough to act as a static wing and can still handle the forces generated when rotating is a serious engineering challenge that will add more weight and complexity (because the rotor hub must have hinges that can absorb the forces when rotating but keep the blade rigid when stationary).
You also need some complex, and therefore heavy, mechanism for tranisitioning the thrust from the rotor whilst lifting to whatever propulsion you use for forward flight.
More weight, more complexity, more maintenance, more cost, more to go wrong. What's not to like? ;)
The other answers also point out better solutions such as vectored thrust and using the same rotors to lift and to provide thrust in forward flight.
Add this all together and you have a solution for a problem which doesn't exist.