When training for bodybuilding, it is important not to compromise form, to ensure that you recruit the maximum amount of muscle fibre throughout a movement. Initially it may feel like you are using too light a weight, but it is better to gradually increase the poundage, without sacrificing form. Some exceptions do exist however, such as using ‘partial reps’ for training.
Even if you are not training for bodybuilding, form is everything. Reduces chance of injury, and insures that you are using the correct muscle.