This is basically what I was trying to do:
try await someImportantOperation()
// success!
async let _ = someAsyncSideEffect()
// carry on without caring about that second task
Which, as established, doesn't do what I initially expected it to (the context would still wait for that second task even if I didn't). But even if it did, given my intent was to ignore it, I didn't feel like that was as well-expressed as through an unstructured Task {}. So I switched back.
If my intent was to stick to strict structure, then of course async let would express that better, not least because I presumably wouldn't be dropping the variable.