The amount of force generated remains the same, however it is applied in a different direction; you therefore get an effective reduction in force for a given axis. To use the two extremes:
- If the airflow is straight down, all of the force is used for lift
- If the airflow is completely horizontal, none of the force is used for lift
Aligning the motor at angles between these splits the force between horizontal and vertical. The 'best' angle for your design of course depends on the type of aircraft; for a rotary wing aircraft you need to generate a lot of lift from the rotors but for a fixed wing you want thrust, while the aerofoils create the lift.
In your image, the airflow would remain the same but the two motors would be pulling against each other - this decreases efficiency, however the advantage can be improved manoeuvrability in flight as rather than just relying on gravity to roll and pitch the aircraft you can use the angled thrust to 'push' the aircraft around.