Unless there's some logical reason as to why this can't happen, I think it would be best if calling $(selector).data(object) would extend the current set of data. That is, if I were to say:

var data = {a:1, b:2, d:3};

$("#id").data("a", "apple")

.data("b", "banana")

.data("c", "codfish")

.data(data)

.data();

The return would be {a:1, b:2, c: "codfish", d:3};