Generally speaking, it is accepted that it is more effective to move through the full range of motion and get less reps using less weight, than to limit your ROM and move more weight and/or more reps. This is the general premise behind most standardised resistance training. Having said that, there are many effective training techniques that employ methods that limit the range of motion (ROM).
The rationale behind this form of resistance training is to bring the muscles being worked from a full stretch to a full contraction (or, as much as is allowed in a particular motion). A good example is a bicep curl. When your arm is straight, your bicep is fully stretched. When you reduce the angle at the elbow as much as possible, the bicep is fully contracted. Therefore, you should go from a straight arm to a full contraction. Basically, this is full range of motion (ROM). The same can be demonstrated with any weight training exercise.
Of course, there are many, many exceptions to the rule particularly if you:
- Are training for a specific reason
- Have an injury
- Have a goal that is more strength focussed
- Are trying to break through a plateau