Claiming that 10%, or 3% or 5% or 8%, of all Southerners owned slaves may be true but it is also a misleading statistic. It implies that since only a tiny fraction of Southerners owned slaves then only a fraction of Southerners drew benefit from them. But it ignores the fact that virtually all the slave owners had spouses and children. Take your 10% figure and add a wife to each and suddenly it becomes 20% who directly benefit from slave ownership. Add children and the figure grows.
A slave had a single owner, that much we can accept. The more accurate statistic would be to compare the number slave owners with the number of registered families in as tallied by the 1860 census, and those results are available on line. If you look at that figure then you see that in Mississippi, for example, 49% of all families were slave owning families. In South Carolina that figure was 46%. In Georgia it was 35%. In all of the original 7 seceding states 36.7% of all families owned slaves. Suddenly slavery goes from the institution of a tiny fraction to one that a sizeable minority drew direct benefit from. And which many more people took an indirect benefit from.
Now I know that not every slaveholder had a wife and children. But the number that did not would not be great enough to throw off these figures by any significant amount. So while it may be easy to ask why the South would rebel over an institution that only 5 or 6 percent of the people engaged in, it's easy to see how they could do so to defend an institution which half the people or more drew direct or indirect benefit from.
Finally, I thought you might enjoy these words, from the final verse of the official State Song of Maryland...
I'm well aware of the lyrics to "Maryland, My Maryland". I also know that the powers that be that run the state dropped those lyrics years ago. Bunch a wimps.
The last attempt was via a House Bill in 2009. It did not pass because, apparently because the Maryland Senate chose to "take no action" on it.