Makes no sense under Unified MMA rules, because each round is scored in isolation. So having a guy carrying an advantage from the previous round wouldn't really align with the scoring system.
If we were scoring under Pride rules, where the entire fight was taken into consideration, then this would make more sense, but it still poses the problem of potentially making bouts less exciting. Not only because people prefer striking, but also because it takes a lot of the incentive away for the fighter with the dominant ground position to work for a finish, because they know they'll just carry that position to the next round anyway, so there's no sense of urgency.