Kant argues that the virtue of any actions is dependent on the underlying motivation, which should be borne out duty as against motivation. It is easy to understand how everybody’s fulfilment of their duty (as opposed to self-interest and other extraneous interests) would lead to morally right decisions. However, the infeasibility of Kant’s suggestion of an abstractly conceived moral law that may guide decision-makers to reach the right decision in every set of circumstances renders this theory impractical. Ethical challenges change every single day, more than it can be envisaged by whomever it is that would attempt making such a moral law. It is neither practical nor desirable to attempt to make moral maxims about every moral/ethical difficulty that people struggle with, even assuming that others agree with them. Difficulty ethical issues today, such as abortion rights, capital punishment and nuclear proliferation, and gay rights involve situations that people/groups make maxims, which, even though driven by good will, are disagreeable to just as many people. This leads to even deeper ethical issues that Kantianism would not solve.
Effectively, for all its flaws, utilitarianism offers a rough-and-ready approach that people can make decisions, in just about any ethical situation that presents that would actually be morally justifiable. Attaching moral/ethical value to the decisions (as against the motivation for the same) means that actions would be right id they proportionately promote greater happiness. Morality itself seeks to ensure that people make decisions that are in line with set values, culture, traditions, and/or principle, etc., which ultimately promotes the common welfare of the specific society as a whole. Effectively, by placing value on decisions that promote happiness, utilitarianism succeeds in achieving the primary goal of ethics. This is perhaps exemplified by the fact that Mill made reasonably accurate calls with regard to liberty and gender inequality in marriage, way ahead of his time.
Bentham, Jeremy and John Stuart Mill. The Utilitarians: An Introduction to the Principles of Morals and Legislation, Utilitarianism, and On Liberty. London: Dolphin Books, 1961.
Britannica. Kant: The Moral Order. n.d. 10 July 2015. <http://www.philosophypages.com/hy/5i.htm>.