If the only function of a cooperative is to better inform the small farmer of market conditions then the you may have a point. Implied in this discussion is that the sole role for cooperatives is to improve market linkage, but there is also the consolidation of farm inputs and outputs to achieve the scale that markets need to be efficient.
Despite all that communication capacity there is still a need to achieve the benefits that scale has in terms of transport, purchasing, etc. Knowing what the maket condition is should allow members of cooperatives to be more effective and better negotiators. Volume does count.
Finally, Cooperatives may provide access to services and equipment that small farmers cannot justify based on their single operation. As scale increases the benefits of cooperatives may decline but it still provides benefits to medium and large scale farmers.