[[:Category:term cleanup/with sc]]
I thought this was still recommended (as it is with {{t}} and so on) as it saves time to specify the script rather than have the server look it up using the language declaration.
That category contains pages that use {{term}} with no language but with a script. So it's not quite what you had in mind. I suppose adding a script when a language is already specified does speed things up slightly, but that is only really recommended in the short term. Once a template is converted to Lua, it will not make a difference anymore. So I really recommend just not doing it, to avoid (possibly confusing) redundancy if nothing else, and also since it's only a short term improvement.
Is it safe to say that these entries need to be cleaned up by hand? If so I'll help out.
All help is welcome, but it would probably better to work on the main Category:term cleanup category first. The subcategories are there because they help to separate out things that a bot might fix from things where it's not as simple.
Shouldn't instances of {term} that leave the first parameter blank be filtered out since they don't create links?
It seemed to me that the two main reasons for making lang mandatory was to link to correct sections and to identify links to terms in certain languages, but correct me if I'm wrong (not a rhetorical device; I'm being sincere). And those two cases don't apply if there is no link created.
But now that I think about it I guess using {term} when there is no language isn't ideal.
If there is no language for a given term, and it's not translingual, then what is it? In any case, though, language codes have a purpose even without linking: they mark the text as being in that language, and select the script for that language. Of course, if the script has been provided explicitly, then there is no reason to automatically determine the script, but the first point still holds. If you write Eisenbahn in a page, then is by default marked as an English word, while Eisenbahn is marked as German. You don't see a difference, but there is a difference in the underlying markup.
Just from cleaning up a few entries I've seen {term} used for "terms" in Germanic (lolwut - maybe you can make sense of it?) and used to link a term to itself as a very roundabout way to just put the pagename in italics.