This is more of syntax sugar than power and generality, since nested pattern matching can be mechanically translated into "top-level" matching (e.g., see [1] and [2]).
[1] L. Augustsson. Compiling Pattern Matching. In Functional
Programming Languages and Computer Architecture, pages 368–
381, 1985.
[2] P. Wadler. Efficient Compilation of Pattern Matching. In S.L. Peyton
Jones, editor, The Implementation of Functional Programming
Languages, pages 78–103. Prentice Hall, 1987.
[1] L. Augustsson. Compiling Pattern Matching. In Functional Programming Languages and Computer Architecture, pages 368– 381, 1985.
[2] P. Wadler. Efficient Compilation of Pattern Matching. In S.L. Peyton Jones, editor, The Implementation of Functional Programming Languages, pages 78–103. Prentice Hall, 1987.