The problem is that an "unused" frequency in one location may belong to a channel that is receivable, with a very good antenna, from another city. If this format becomes widely used it could kill DXing and the viewing of distant border stations.
Yeah, that may be a concern for a VERY small group of antenna users, but it's same as for any WSD usage near that particular user. Have you EVER heard of local a WSD interfering with DX activities?
Given the extraordinarily shallow usage of Super Wi-Fi and no evidence that its use is expanding, the odds of it happening to anyone who could definitively pin it down appear, to me to be extremely small.