America was born from an act of secession. When King George's rule trampled on the rights of the colonies, we successfully seceded from England. It took a war, but we were well within our rights. We applauded when former soviet states seceded from the USSR and declared their sovereignty. And hopefully the United States will eventually secede from the United Nations. We pay most of the bills of the UN, yet do not have the commensurate votes, so someday we will wake up and realize that membership, for these and other reasons, does not serve our interests.
On a personal level, contracts you enter into can be terminated if one side unilaterally changes the terms. If a credit card company jacks up your interest rate, you have every right to fulfill your obligations and close the account. Imagine if you were forced to stay with a credit card company forever no matter what just because you previously signed up! The principle of self-determination applies to political unions as well. In the cases I mentioned above, governing organizations transformed into much more overbearing entities than originally agreed upon. Several state constitutions originally had clauses explicitly allowing them to opt out of the Union down the road if they so chose. I doubt our country would have ever come together if this were not the case. Just because the north successfully kept the union together by force with the Civil War does not mean that enslaving the states is a legitimate alternative.
Secession is the last resort of states whose sovereignty is over-ridden by an overreaching federal government. The federal government has only itself to blame for this talk. Recently, some states have enacted laws allowing for the medicinal use of marijuana, yet these laws are basically voided by the continuing raids by the DEA, sanctioned by the administration. The federal government is also strong-arming states with stimulus money, forcing them to expand programs they know they will not be able to afford in the future, at a time when many states' budgets are already in the red. This is not a new problem. No Child Left Behind burdened the states' education systems and forced them through many hoops designed by federal bureaucrats in distant Washington DC rather than allowing communities to tailor education to their children's unique needs. There are numerous other examples of the erosion of state sovereignty and many governors are frustrated, not just ours in Texas. Without the right to secede, state's rights are meaningless.