If you have reproducible builds it's no different. Without those binaries are a nightmare in that you can't easily link a given binary back to a given source snapshot. Deciding to trust my upstream is all well and good but if it's literally impossible to audit them that's not a good situation to be in.
I think it’s already probably a mistake to think that a source distribution consistently references a unique upstream source repository state; I don't believe the crate distribution layout guarantees this.
(I agree that source is easier to review and establish trust in; the observation is that once you read the upstream source you’re in the same state regarding distributors, since build and source distributions both modify the source layout.)
