Jeremy Bentham argued that animals deserve moral consideration on the grounds that the ability to experience pain and pleasure was the proper basis for moral consideration rather than the ability to reason. As he put it, in an oft-cited line: "the question is not, Can they reason? nor, Can they talk? but, Can they suffer?"
But what if a being cannot suffer? Given Bentham's utilitarianism, the ability to experience pleasure and pain (sentience) was not simply a sufficient condition for moral considerability, but also a necessary condition.
This would mean that butterflies, corpses, and individuals in persistent vegetative states (who lack all traces of sentience)--just to give a few examples--are not owed any direct moral consideration. To put it another way, such beings can't be wronged. Is that right? Some will suggest that even if we can't "wrong" a butterfly, it's still wrong to pull the wings off a butterfly for our own delight because this shows a callous character. Similarly, we should not degrade corpses because doing so would offend others (and we might also be under an obligation to respect the implicit or explicit wishes of the deceased, though this justification probably falls outside the scope of a utilitarian position). The utilitarian might offer a similar line about people in persistent vegetative states.
But in the case of the butterfly, the justification for why one shouldn't pull off its wings is purely contingent on such behavior tending to produce a bad character. Robert Nozick has wondered why we should expect such behavior to do so, if we are able to maintain a clear understanding of the differences between insects and, say, humans. Furthermore, if a person did take great delight in dismembering butterflies and did not allow this to affect his behavior toward humans and other beings that are morally considerable, then the butterfly dismemberment would have positive hedonic value on the whole, since the person doing it enjoys the activity.
In the case of corpses, what becomes clear is that utilitarians can't make any room for the notion of "respecting the dead." This is because we can't wrong the dead; corpses can't suffer. Perhaps we must respect those who care about the dead by treating those corpses in ways that don't increase the suffering of the living, but that isn't the same as respecting the dead. In principle, we may do whatever we please with corpses on a utilitarian view. While it's true that the conventions and treatments by which we pay respect to the dead vary amongst cultures, it doesn't strike me as plausible that the moral reason why violating the relevant traditions is wrong is because such violations upset other people. That is, the wrongness would seem more plausibly to have something to do with the failure to show respect for the dead.
If this is right, then other things matter besides welfare, and while sentience is surely a sufficient reason for seeing a being as warranting moral consideration, the fact that something cannot suffer does not mean that it counts for nothing, or that what we do to it can only matter, morally speaking, in some indirect way.