WMS GetFeatureInfo responses can be customized by editing the default XSLT templates. However, this will affect all WMS services on the server. The ArcGIS documentation does therefore discourage users from putting service-specific logic into the templates and presents the XSLT_TEMPLATE parameter as a client-side alternative.
In my opinion, separate XSLT templates for each service would add flexibilty while style control remains with the service author.
Use case:
A service author might use a service-specific XSLT template to convert attribute values representing a URL to an HTML hyperlink. They could apply this transformation to field "URL" in the case of service "A" whereas they could apply the same transformation to field "RESOURCES" in the case of service "B".