Suppose there is a paper with 3 promised reviews, 2 of which have been submitted. In this scenario there will usually be no decision while the editor waits for the third review. Will the author want to see the submitted reviews before a formal decision is made?
Intuitively I'd guess "yes", especially if the decision turns out to be "revise", but I am not so sure if the decision turns out to be accept/reject. It could be jarring for the authors to see reviews with minor comments, and then receive a scathing 3rd review & a reject decision, for example.
Another possible issue is if the 3rd promised review never arrives, which from the editor's perspective will inevitably happen for some papers, but this seems less likely to aggravate the authors.