If the SC asks for more material it means that someone wants to read more of what you have written. That is always good. (While it may be "cheap" to ask for more material, it is never cheap to read more material.) As pointed out above, this certainly means that you are on a long-short list, and the school is narrowing to an interview list. Unlike ^^^^, my department has never felt constrained by a rule that "what we ask of one, we must ask of all." We regularly ask some applicants for more material. This is especially true for dissertation work where we want to get a better sense of the theory or empirical chapters when only one of these was sent, or when we want to get a better feeling for how far along the dissertation is.
You have probably led with your best work, and may be hesitant to other papers/chapters. That's your call, but at least for me, holding back is a bad idea. We understand what dissertations look like in progress, are are probably not as critical of these chapters or working papers as you are. Ignoring or deferring the request will more than likely keep you at the long-short list. Sending more material might do the same, but also might help move you to the interview list. Since being asked to send more work is a good sign, and probably leave you no worse off than you are now, my recommendation is that you send promptly.