Tamas pulls back the Rajas when it goes overboard. It is what induces in creation the urge for rest and sleep, without which there would be, for most human beings, a breakdown due to the overrunning and overexcitement of the system. After all, the entire animal world is maintained by the balance between the forces of rajas and tamas. The Sattwa element maintains the balance but is hidden behind. Conscious Sattwa emerges primarily in human beings.
Yes, excess of tamas is harmful, but so is excess of rajas. Even an excess of Sattwa without the energies of Rajas motivating the human soul towards new conquests is also to the detriment of our integral progress. The earth (tamasic), the winds and waters (rajasic) and space (sattwic) must all be balanced and brought under the spiritual element, the fire within us.
Affectionately,
Alok Da
