It would be particularly useful to map templatedata between templates, especially between different wikis (and specifically language versions of the same project).
Apart from giving a picture of the different features of a template in various versions (and saving work on adding templatedata), it would make it possible to "translate" a template from a language/version to another. Adjusting template calls is probably the most tedious part of article translation; usually templates are just left as they are (in English), whether fully working or not, or removed.