I think this is a good question, and if I may let me offer a different set of semantics from "conservative" and "progressive". Instead, I think the labels "traditionalists" and "reformers" may be more precise and avoid others from reading their own bias towards those terms into the debate.
Secondly, I would not put slavery as an issue that falls on this spectrum. The reason is from the founding, slavery was one of the most divisive issues the young nation faced. The 3/5ths compromise/clause is an example of this. This was not, as students are taught in High School and American/Black Studies, a scheme to say that blacks are 3/5ths of a man. It was a check on Southerners from using their slave population to gain seats in the House of Representatives and the Electoral College. Then you have the Missouri Compromise which basically meant that for every 'free' state admitted to the Union a 'slave' state would be too (and vice versa). So the point is, slavery was unpopular from the beginning with many not wanting it since it violates our fundamental belief in individual liberty.
But I digest...
As for my answer to the question: traditionalists act as a necessary foil to the zealousness of reformers. Look at the French Revolution...left to their own devices reformers will devolve into reigns of terror to protect their grand notions of freedom, liberty and equality. Even if it means to destroy those principles. Traditionalists who seek to keep instiutions strong as well as preserve the good of the past, allow civil society to move forward more slowly and rationally instead of quickly jumping in before testing the water for depth.
My example would be hate laws. I support gay marriage and gays in the military. I support equal pay for equal work and equal access to opportunity. I do not believe that sexuality, gender, religion, race, etc are legitimate forms of discrimination. (NOTE: there are legitimate forms of discrimination. Not being able to see well enough should keep you out of the cockpit of a F-16 or not having a MD and medical training should prohibit one from being a surgeon.) But I do not believe that there should be 'hate crimes' legislation. Snuffing out a life, regardless of motivation, should be punished equally or else the rule of law becomes cheap and unequal. Having Constitutional Traditionalists in a republic, keeps those whose politics are ruled by emotion in check.