Nobody says they are useless. Each thing has its own place and purpose. Besides there are scriptures and scriptures. Not all sages reached the same height just like different scientists giving us different types of therapy. The power of the mantra comes from the level of consciousness to which the Rishi had arisen and to regard all as one and the same is as irrational as saying that all religions are the same. There is no end to the Divine Revelations and the Vedas are not a finished work whose last word has been written.
Even within the Vedas there were distinctions made between the mantras that lead to material gifts and the mantras that help the soul towards freedom from Ignorance. Sri Krishna, in the Gita goes so far as to say that one who has surrendered completely to God needs no scripture and bids the seeker to go beyond all scriptures, written or heard, sabdabrahmativartate.
We can arrange the entire system of Vedic Knowledge and the mantras in a hierarchy of needs from the most material to the highest spiritual. But we should also remember that the Divine is always greater than any scripture and mantra, else we shall become slaves of a book and its interpretation instead of being slaves to God.
Affectionately,
Alok Da
