The belay loop is designed for rappelling and belaying only. Use it if you find it convenient. If you tie the rope directly into it, you will wear it out very quickly. On multi-pitch climbs, it is important to have the forces of the belay link directly through to the anchor, usually by clipping the belay biner into both the belay loop and the tie in loop of the webbing.
Understanding the forces involved in climbing is helpful in climbing safely. Understanding the limitations of your gear is essential.