While reviewing first-posts and late-answers in Stack Overflow I find that I very often have to click through to the original post so that I can see existing answers to the question. This has been brought up in passing a couple of times ( Why don't questions shown in the review section show votes/answers/views like the rest of Stack Overflow? , reviewing answers, when answers are correct but duplicates) but hasn't received any satisfactory discussion or answers.
This seems to be a particular issue when reviewing answers... Anecdotally, I've that a significant percent of late-answers and first-posts that are answers look good when viewing the question and the new answer, BUT if you click through to the whole post you find that the answer is just a repeat or summary of answers posted earlier by other users. Give that SO clearly discourages this type of behavior, we should be downvoting/commenting the answer, but because we can't see existing answers at a glance (we can't even see if there are any existing answers) it's easy to miss this and give it an upvote or pass it by.
I'm of the opinion that you really can't judge the quality of a new post on SE without seeing earlier posts (at least for answers). If there's a four paragraph point-by-point explanation of a solution and someone comes by a year later and gives an answer that just rehashes the same answer (often with less detail), then the new answer doesn't deserve upvotes and the poster should be alerted that they should only post an answer if they have something to add.
Is there a reason to not allow the whole thread to show when reviewing an answer?
EDIT: Even more reasons to show answers to reviewers
It's been a few days since I posted this, and I've come across another common scenario which would be much-improved by having easier access to answers - this time concerning questions. I came across this question in the course of reviewing close votes. I couldn't tell what the OP was asking and I almost added the fifth "Not a real question" vote. However, when I opened up the full page to have a look, it appears that that another user understood perfectly and answered the question to the OP's satisfaction. In light of the answer it was obvious what was being asked. In this case, the OP was misusing a couple of words that made it hard to tell what he wanted to do and so I went ahead and edited the question to clarify. In the normal course of reviewing I would have probably closed the question as "not a real question" had I not seen this answer.
As it turns out, I ended up casting the closing vote anyway, but I cast it as a duplicate instead of
not a question because once the question became apparent it also became apparent that the OP likely failed to find a solution because he didn't know the words to use to look it up. But this was the correct outcome. The question should have been tweaked and closed as a duplicate, not as "not a question". And there are cases where this same scenario unfolds but the revised question has merit in its own right, in which case showing the answers help prevent the closing of salvageable questions.
As the number of reviewers grows, the chance that five given reviewers might not understand a question that is comprehensible to others grows. Showing us other users' answers helps demonstrate when a question needs to be clarified (edited) vs closed as not a real question.
And yet more reasons to show answers to reviewers
Another scenario which I've seen a number of times going through the
close review queue is where a seemingly legitimate question has been asked, with lots of details provided and relevant code posted, but the close vote is on "Too Localized" grounds because the OP subsequently posted an answer to the effect of "Oh, my bad, the error had nothing to do with this code and is because i didn't upload my changes to the server" - or some such thing. I probably come across this every other day, and not showing me posted answers off the bat creates a barrier to investigation and encourages lazy reviewing. At the very least, on questions up for closure on "Too localized" grounds where the OP has posted an answer, it should automatically show us the OP's follow-up post.