From personal experience, as well as seeing other developers/designers needing this, I've noticed a real need for multiple repeatable zone (or ability to add groups) in Slices? Some section designs are impossible to be done using the current Slice setup and need to resort to implementing "hacks" which are far from optimal. When I've asked this in the past, people said that it's currently not possible to do this but they might be looking into this in the future if there is a need.
Given that Slice Machine is an ongoing work in progress, it might be a good opportunity to make this change to the API. But I might be missing some limitations from a technical standpoint.