Does it make any sense to change phutil_build_http_querystring to implement this this "if it's an object or an array, turn it into a JSON string" behavior?

I think this is too magical to put in phutil_build_http_querystring(). I like that we're moving toward a very dumb, no-assumptions implementation of query string behavior in D20136, and think that's generally the right direction: the basic primitives shouldn't have any implicit magic.

It would be reasonable to put in some other wrapper, but I think this pattern is relatively rare, and we're often building the JSON payload in Javascript when we do use the pattern (e.g., to save a Herald rule). Worth keeping an eye on and it might happen more often in the future if Ajax stuff gets fancier, but I'm hard-pressed to think of even a second callsite offhand (but maybe other uses are just slipping my mind).

In trying to come up with examples, I think when we do stuff like this (pass complex data from one PHP endpoint to another PHP endpoint without using a form), it's usually passed to the Javascript layer as Javelin metadata (which does get implicitly JSON'd / un-jSON'd) and then passed back to us by Javascript, often after modifications. A recent example is that timeline rendering change to Differential in D19918 or some followup (where Differential needs to pass which diff you're looking at to the timeline rendering code, so it can link inline comments either to anchors on the current page or to other pages).