Whereas theoretically.it sets up a single test with a varying value, theoretically.describe would set up an entire section with a varying value:

describe("NumberStack", function() {
  theoretically.describe("empty()", [
    emptyNumberStack,
    numberStackOneElement,
    oneHundredElementNumberStack
  ], function(numberStack) {
    it("removes all elements", function() {
      numberStack.empty();
      expect(numberStack.length).toEqual(0);
      expect(numberStack.pop()).toBeFalsy();
    });
  });
});

(This may not be the best example as it only has a single it in the theoretically.describe. What would be a good example, where several small tests would use the same varying value?)

What are pros/cons of this feature? Should it be added?

Technically this should support mocha, jasmine and jasmine-node. This should be tested and documented.

I have this case;

describe("constructor", function() {
    it("fails when at least one object doesn't pass validation", function() {
        var constructor;

        constructor = function() { new Item("something wrong", "something wrong"); };
        expect(constructor).toThrow();

        constructor = function() { new Item(goodFirstArgument, "something wrong"); };
        expect(constructor).toThrow();

        constructor = function() { new Item("something wrong", goodSecondArgument); };
        expect(constructor).toThrow();
    });
});

How could this be handled through jasmine-theories? Maybe adding some functionality that creates permutations of input might be prudent?