The ban has its roots in pastoral, and later, agrarian societies that grew along the Gangetic plains. Though the region was fertile, floods and droughts were frequent. When a drought strikes and grains become scarce, the natural tendency is to kill animals for food. Besides, water requirements of animals are high. So, it is extremely difficult to maintain domestic animals during a drought.
However, if massive killings of cattle took place, it would destroy even hopes of revival of the economy after drought. When tractors and other machinery were not there, draught animals were critical for revival of agriculture. While killing of cattle en masse could not be stopped, it was important to save at least the cows. If cows are there, even a single surviving bull can sire many calves. (Even now, a practice is there in North India of offering a bull to the community with connotations of bringing prosperity for the donor).
Some wise men apparently thought that the best way to effect conservation of cattle was to formulate it in the form of a religious edict. Obviously, this edict has lost its relevance today because we have better methods of farm management including artificial insemination and modes to transport grains across large distances in case of a regional drought. (Note that hauling loads of grain too required draught animals in olden days.)
Now, cow is no more central to the economy. However, certain other religious beliefs like those surrounding sacred groves in Kerala, for example, retain their relevance even today. Cattle also have role to play in organic farming even today, though that does not negate their slaughter.
It is also worth examining whether vegetarianism has any advantage over consumption of non-vegetarian food. Evolutionary history shows that Homo sapiens were never strict vegetarians. They started as hunter gathers and are omnivorous. Besides, a 100 per cent vegetarian dish can deprive you of some essential nutrients. Milk products and egg can make up for that to some extent. (So, here also cow becomes important). However, it is to be noted that in some regions of the world, you have to be heavily dependent on meat because of scarcity of vegetables and grain. In the arctic region, no vegetable is available locally. And people like the Eskimos, Inuits and the Cree traditionally survived entirely on meat. (They too have beliefs that ensure survival of animal species. The Crees have a method of determining whether the animal is “willing” or ready to be killed. If not, they will spare it.)
However, vegetarianism reduces your carbon footprint. More resources are needed to produce meat compared to vegetables. This is why even U. S. President Barack Obama is concerned about increasing meat consumption in Asia. He wants Asians to eschew meat consumption to keep American dream alive. So, RSS and the US have common interest though for differing reasons.
As human cultures and technology progress, it is expected that cruelty against animals will come down irrespective of whether it is a cow or bull or a goat. However, one should not be creating communal divides over the issue and commit bigger cruelty against fellow human beings.