Well, I'm quite serious when I say that I don't think crates should ever need to ship binary artifacts, and furthermore that the social contract of crates.io is that it serves you source and only source, with the possible, grudging exception of prebuilt core Rust toolchains, which you get via a different mechanism than anything else (rustup instead of cargo). If I caught a crate in my software's dependency graph shipping a binary artifact I would drop that crate and everything else written by the same people from my dependencies and never use them again, that's how strongly I feel about this.
You say that "many" crates are doing this. Please give us your list, and, if you can, explain why the authors of each crate on the list think they need to do this.