If one of the factors is strengthened at the expense of the other, the balance of the mind is lost and meditation impossible. Concentration and mindfulness are distinctly different functions. They each have their role to play in meditation, and the relationship between them is definite and delicate. Concentration is often called one-pointedness of mind. It consists of forcing the mind to remain on one static point.
Concentration is pretty much a forced type of activity.
It can be developed by force, by sheer unremitting willpower. And once developed, it retains some of that forced flavor. Mindfulness, on the other hand, is a delicate function leading to refined sensibilities. These two are partners in the job of meditation. Mindfulness is the sensitive one.