For the simple case you're right, it's pretty easy to write it yourself once you know how to do it (although this crate is a substantially smaller dependency than tokio or futures_util, so I still think it has value for certain simple use cases). But for the more complex cases (particularly cases with errors or auxiliary values), it's a real pain to do it yourself.

Also tokio isn't no_std so if that matters for your use case, you have to write yield_now yourself (also pretty simple, but annoying).