In Aaron's original proposal for this support, he mentioned the need for overscan compensation and this being needed by NVIDIA's Tegra driver. "I'm trying to add support for overscan compensation to the Tegra X driver. This driver uses RandR 1.2, so I need to add an official mechanism for X clients to describe the desired configuration through the protocol."
Other open-source X.Org drivers have handled this in other non-standard ways, but now Aaron has provided a new patch that adds helpers to RandR to allow the DDX to specify how many dimensions of adjustments it supports.
The border property support would be part of RandR 1.4 extension and likely integrated into X.Org Server 1.12. RandR 1.4 was supposed to be part of xorg-server 1.10, but was then yanked and hasn't yet returned. RandR 1.4 should also be special in that NVIDIA's desktop Linux driver may finally support it thanks to changes made, but first we need to see RandR 1.4 appear in released form.