There are some tasks you shouldn't leave any old carpenter - among them, the function of your doors and windows.
At stake if hardware is improperly installed:
Doors and windows are the moving parts of your building - they need to survive daily abuse. Properly installed hardware can save years of unnecessary maintenance and energy costs.
If you have two different companies doing your finish carpentry and painting, you are necessarily sacrificing money or quality - probably both.
By having one company handle all of your finishes, you streamline the last phase of your project by eliminating any possibility of conflict - scheduling or otherwise.