p.s. Actually now I do see the usage in DrRacket -- both arrows and docs (in the upper right corner) for #%app, as well as for #%datum.
This is definitely used.
So I'll try supporting it.
If the disk space usage seems too big, I'll think about ways to represent it more compactly, as well as the gzip-ing that I'm already doing.
p.p.s. We had discussed this previously when check-syntax started doing this a couple years ago; I should have remembered sooner, and searched around for that earlier.