Specific terms that offer pro rata re-assignment at YC discretion:
"Neither this Agreement nor the rights contained herein may be assigned, by operation of law or otherwise, by Investor without the prior written consent of the Company; provided, however, that this Agreement and/or the rights contained herein may be assigned without the Company’s consent by the Investor to any other entity who directly or indirectly, controls, is controlled by or is under common control with the Investor, including, without limitation, any general partner, managing member, officer or director of the Investor, or any venture capital fund now or hereafter existing which is controlled by one or more general partners or managing members of, or shares the same management company with, the Investor."
I think this is exactly why the post was written. Aaron isn't saying that YC has a great pro-rata program. I took his writing as stating that he thinks it needs to be reconsidered, and that it is bad as it currently stands.
Why? It has to do purely with the fund's internal structure. Affiliate transfers shouldn't have any material effect on a company. Withholding automatic affiliate transfers would be the company creating needless work for itself.
"Neither this Agreement nor the rights contained herein may be assigned, by operation of law or otherwise, by Investor without the prior written consent of the Company; provided, however, that this Agreement and/or the rights contained herein may be assigned without the Company’s consent by the Investor to any other entity who directly or indirectly, controls, is controlled by or is under common control with the Investor, including, without limitation, any general partner, managing member, officer or director of the Investor, or any venture capital fund now or hereafter existing which is controlled by one or more general partners or managing members of, or shares the same management company with, the Investor."