If you've got enough voters to make repeal happen, you've got enough voters to elect politicians who support states' rights and can do so directly, without all the problems that come with introducing a Constitutionally-required electoral middle-man. This 51% of voters can vote out politicians who don't properly respect states' rights, rather than depending on 51% of legislatures to do the same.
Term limits can be decided similarly. Instead of slapping a hard limit of 8 years on a Presidency, voters can decide when new blood is required. I'm all for an argument about whether Obama should be elected for a third term, but voters should at least have the option. If enough of us think Obama needs to go, whether it's because we believe that two terms is enough or that he's a anti-colonial Marxist, he goes. If a majority think he should get another term, the nation isn't deprived of four years of what that majority thinks is the way to go.
Why should we let people in 1951 strip us of the right to vote for four more years of Reagan or Clinton (or Bush, I guess...)? What tremendous insight into the effects of eight versus twelve years in the White House in the 21st century did they possess then that we can't trust 51% of us to recognize now?
Let the voters decide.