It will depend what you mean by usefulness. Some of it is a bit rubber chicken, going to open festivals, etc in the constituency but that keeps them in touch with their constituents, lots will be correspondence, lots will be either sitting on it preparing for committees or special interests, they take up. One person I know spends time campaigning against the death penalty around the world, dependent on one's views that might be useful or not. I don't think there is an easy description of useful and suspect that it all depends on one's perception of that. But it is not an easy job if done right. Yes, there are ideological aspects, such as party events but given the system, that is part of the job too, and I don't see a way of avoiding it. You also can't plan to have free time if you are doing anything in your constituency such as shopping, as people will recognise you and bring you their issues.