It is useful, because I believe (without being able to prove it) that people have some kind of inherent worth, not contingent on the scale of the contribution to society that we (using our own system of measurement) perceive them to be making, which I would struggle to describe or quantify but am wary of accidentally discounting; because I also believe that (by and large) people have to decide for themselves what motives or morals guide them; and because for me to make these unstated assumptions about other people comes perilously close to projecting my own value system on to them in a way that I would prefer not to do.
However, it is also disheartening, not because I want other people to believe or act differently, but because of the effect their stance has on my confidence in my own position. I feel that I could and should (and in some cases even do) contribute to society in excess of the gift of my existence, and yet I am often frustrated by how little I do contribute; so to see other people explicitly disavowing any interest in giving or doing any more, and being content and happy (which are two different things) in that position, makes me wonder if I am going about things the wrong way. Not that this is necessarily a bad thing to wonder; but it deflates, it undeniably deflates.
To every sentence accrues a thousand unstated assumptions.