I noticed a site using this tag:

<link rel="search" type="application/opensearchdescription+xml" title="XXXXXXXXX" href="http://www.XXXXXXXXXX.com/api/opensearch" />

As I understand it (based on http://www.opensearch.org/Home), this tag is a way of describing search results (so you use it on pages which contain search results) to make it easier for other search engines to understand and use your results.

Given that Matt Cutts has said Google generally frowns on "search results within search results" is using this tag a bad idea on a page that you hope to achieve a good ranking in Google?