A second passport is another tool in a your toolbox. Maybe it will never have a use, but maybe you'll find yourself in a situation on day where you're glad you have it. Given how little a passport costs, there's no reason not to do it, assuming both countries allow dual/multiple citizenship.
To that end, South Africa does not by default allow dual citizenship, rather, as soon as Home Affairs are notified that a South African citizen has been granted citizenship by another government, Home Affairs will revoke your South African citizenship immediately. You can apply for a "retention of citizenship" certificate which will exempt you from this rule. It's a trivial process - fill in the form, hand over some cash, and you get a certificate that says you're allowed to retain your South African citizenship while acquiring another.
Lastly, be aware that it is a criminal offence for a South African citizen to enter the country on a foreign passport (it's fairly common around the world). As far as I know the rule isn't enforced to vigorously, but you never know when this might be turned into a weapon.
