Turning the objection against itself
It's a nice twist that philosophy gave us utilitarianism, which is the basis of your question. And even if it hadn't, the question whether an activity or inquiry is answerable to 'usefulness' and only to usefulness is itself a philosophical question. (What other type of question could it be ?)
Some suppose that philosophy has practical relevance in delivering or vindicating moral principles or in providing political prescriptions. Or in helping us to get analytically clear(er) about the dilemmas that beset ethics committees in their all too practical decision-making. Well, it may be so.
But I feel safer on different ground. It strikes me that whatever activities or inquiries one is engaged in, questions naturally arise that are philosophical. A historian will find it hard not to ask herself at some point in what sense if any the past, which she ostensibly examines, exists. The present might exist, the future doesn't - does the past? And if the past doesn't exist, it can't be known by the historian. Or if it can be known, then in just what sense does it exist and what kind of knowledge is involved.? Not knowledge by acquaintance, for sure. A historian may spend much or little time on such questions but they they will insinuate themselves as puzzles or paradoxes into her thinking, almost certainly, at some stage. Their 'usefulness' is irrelevant. They are presupposed to what she is doing and they will strike a reflective mind at some juncture.
In the law and in everyday practical matters we hold ourselves and others 'responsible' for what we are doing or have done. There are criteria of responsibility in various jurisdictions and societal norms determine, usually less than rigorously and consistently, who's responsible for what in the ordinary way of things. It doesn't take much reflection for questions to obtrude themselves : responsibility involves causality but what concept of causality is in play ? Does it involve determinism and exclude free will ? What is it or would it be for someone to have free will ? And so on and on. These questions might or might not have practical utility but they are philosophical questions that break in on, I'd say, any mind that thinks and ponders - natural dispositions for some or many of us whatever their usefulness or lack of it.
History, law, the ordinary business of life. My few examples can be multiplied indefinitely.
Countering the counterarguments
How to answer people who would claim that "philosophy is useless"?
"Philosophy, since it by def. considers fundamental matters, such as existence, cannot be rationally neglected, because the whole existence of the subject (you) is itself a philosophical issue. Thus the very moment of your current existence is questionable."
However, there could be counterarguments such as:
"I don't find this relevant."
"I don't understand it."
"Philosophy doesn't interest me."
If you take my approach, these counterarguments don't work. They are fine against people who try to press philosophy on you as something in which you should be interested. But on my approach, philosophical questions arise within one's own activities and inquiries. If, say, a historian, reflecting on her activity or inquiry, is struck by puzzles about the past - e.g. (and crudely) 'how can I know the past when it doesn't exist ?', the counterpoints 'I don't find this relevant' (but it has arisen as relevant to her), 'I don't understand it' (she don't understand her own question ? interesting). 'Philosophy doesn't interest me' is self-refuting since she has started doing philosophy in posing her question about the past.