A utilitarian would endorse a non-utilitarian value system if doing so maximized utility.

A utilitarian would endorse a non-utilitarian value system if doing so maximized utilitarian utility, which is really the crux of the debate.

The word utilitarian is thrown around a lot here without clearly defining what is meant by it but I would guess that most of the non-utilitarians (like myself) here take issue primarily with the agent neutrality / universality and utility aggregation (whether averaging, summing or weighted summing) aspects commonly implied by utilitarianism as an ethical system rather than with the general idea of maximizing utility (however defined).