It's not required by any other part of Xfce. ;) 

I seem to recall there was some reason we didn't install it by default, but I can't recall what that was. 

Adding Christoph here.

I only remember we didn't install xfce4-icon-theme because it is broken and unmaintained, but for the gtk-engine I think it should definitely be in the Xfce group. I even think it should be 'default' rather than 'optional'.

... unless I didn't miss something fundamental here.

ok. Shall we add it into comps? or add a dep somewhere? 
comps would make more sense, since there's no real dep on it, but then existing installs wouldn't get it.

I don't think this is a problem and comps is enough.

ok, how about: 

diff --git a/comps-f15.xml.in b/comps-f15.xml.in
index 97a0165..de969ba 100644
--- a/comps-f15.xml.in
+++ b/comps-f15.xml.in
@@ -6779,6 +6779,7 @@
       <packagereq type="default">xfce4-power-manager</packagereq>
       <packagereq type="default">xfce4-session-engines</packagereq>
       <packagereq type="default">xfwm4-theme-nodoka</packagereq>
+      <packagereq type="default">gtk-xfce-engine</packagereq>
       <packagereq type="optional">parole</packagereq>
       <packagereq type="optional">ristretto</packagereq>
       <packagereq type="optional">thunar-media-tags-plugin</packagereq>
diff --git a/comps-f16.xml.in b/comps-f16.xml.in
index dca75c0..c93f1e8 100644
--- a/comps-f16.xml.in
+++ b/comps-f16.xml.in
@@ -6804,6 +6804,7 @@
       <packagereq type="default">xfce4-power-manager</packagereq>
       <packagereq type="default">xfce4-session-engines</packagereq>
       <packagereq type="default">xfwm4-theme-nodoka</packagereq>
+      <packagereq type="default">gtk-xfce-engine</packagereq>
       <packagereq type="optional">parole</packagereq>
       <packagereq type="optional">ristretto</packagereq>
       <packagereq type="optional">thunar-media-tags-plugin</packagereq>
diff --git a/comps-f17.xml.in b/comps-f17.xml.in
index dca75c0..c93f1e8 100644
--- a/comps-f17.xml.in
+++ b/comps-f17.xml.in
@@ -6804,6 +6804,7 @@
       <packagereq type="default">xfce4-power-manager</packagereq>
       <packagereq type="default">xfce4-session-engines</packagereq>
       <packagereq type="default">xfwm4-theme-nodoka</packagereq>
+      <packagereq type="default">gtk-xfce-engine</packagereq>
       <packagereq type="optional">parole</packagereq>
       <packagereq type="optional">ristretto</packagereq>
       <packagereq type="optional">thunar-media-tags-plugin</packagereq>

Please make sure to keep the alphabetical sorting order. The rest is ok.

Done.