Over at Prawfsblawg, Paul Horwitz has an interesting post on the usefulness of the 19th century Mormon cases for teaching law and religion. In Reynolds v. United States 98 U.S. 145 (1879), the Supreme Court upheld the constitutionality of Congress' statute forcing Utah to outlaw polygamy (a practice then sanctioned by the Mormon faith) ; later cases dealt with other legal sanctions imposed on the Mormon church as a result of its support for polygamy.
In my view, the Mormon cases also have an underappreciated usefulness for teaching the history of American constitutional federalism. For example, under the 1894 Utah Enabling Act, Utah was forced to ban polygamy "forever" as a condition of getting statehood and was thereby denied the power to control its marriage law, a field that was then generally held to be outside the authority of the federal government and subject to exclusive state control.
Americans tend to think of federalism as antithetical to minority rights because of the history of local minorities (such as African-Americans in the Jim Crow South) being oppressed by local majorities. Utah, however, represents an important case where a minority at the national level achieved majority status within one would-be state (Utah was still a territory when the Mormons first settled there) and tried to protect its values by controlling that state government. Their experiment was, of course, cut short by federal intervention that undercut the state's autonomy.
Although relatively uncommon in American history, situations where minorities at the national level are majorities within a particular state or province are frequent in other federal systems. To take just one case, Iraqi Kurds are a historically oppressed minority in the nation as a whole, but a majority in the three northern provinces. Not surprisingly, they favor a high degree of decentralized federalism in the new Iraq. The history of the Mormons in Utah is the best example of a similar phenomenon in American history.
The Mormon migration to Utah in order to establish a state under their own control is also an important example of voting with your feet, a crucial advantage of federalism that I have often emphasized (e.g. here and here). There is, however, an interesting twist to the Mormon case, in that they were not trying to migrate to a preexisting jurisdiction that was relatively more tolerant of their religious practices, but seeking to create a whole new jurisdiction where they would be in the majority.