XML is great if you are planing of reusing objects you can inherit the display and the values of XML-based frames to Lua- or other XML-based frames, you can't inherit frames created in Lua to other Lua frames.
So it can be useful to make your templates in XML and using them in Lua. I can imagine that a static XML template will reduce loading time 'cause they won't be interpreted by Lua and this is a important factor of big scaled AddOns : loading time after a ReloadUI.
I had used the XML+Lua construction in my AddOns, because there are WYSIWYG-XML-Editors for WoWXML (i'm lazy
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