As has been previously said, the SRM doesn't specify jhatka for a Sikh. A Sikh can technically eat anything they want as long as it's not halal. The discerning Sikh will read and absorb Gurbani and learn that moderation is good, and that some foods are more appropriate than others (stick to simple diet, don't eat too much sugar, etc). I don't know about you but I've got a long way to go with all that yet!!!
On the other hand, I was talking to a colleague some time ago about obsessive compulsive disorders and the ritualistic behaviours that go along with it. My colleague explained to me that it's more difficult to eradicate a behaviour - it's easier to replace one behaviour with another. Now I'm not saying Sikhs, Hindus or Muslims have OCD (not saying anything about wudu, hehehe, just kidding), but when you come from a history of there being ritual and rules around food, it may have been difficult for some people to just abandon a food-related behaviour, so jhatka was encouraged as an alternative, perhaps? I don't know, just thinking out loud.