Susan G. Kleinmann writes ("Bug#4263: dpkg-source requires patch"):
...
> # dpkg-source -x sgmlspm_1.03ii-2.dsc
> dpkg-source: extracting sgmlspm in sgmlspm-1.03ii
> dpkg-source: failure: exec patch: No such file or directory
> dpkg-source: failure: patch gave error exit status 2
...
patch is only required by dpkg-source, and not to install packages.
I've added it to Suggests and mentioned it in the Description. This
will be in 1.3.10.
Ian.