Letting clients choose the fields they want to receive leads to the awkward realization that any generic, flexible and future-proof mechanism for doing this leads to system where a client can suck down an entire website with a single request.

(If clients can choose not to receive some information, then by symmetry, they should also be able to choose to receive some additional information that's not included by default. And since everything is linked to everything else (orders are linked to users, etc.), you end up with a single resource that potentially embeds everything else.)

These systems also break caching, of course, and also to some extent the principle that within-server links are indistinguishable from cross-server links. The web is not optimized for performance or file size.