Ideally, every post stands alone.
In practice, other answers can get deleted, so even when you review it - consider encouraging newcomers, editing to improve a first post and otherwise being willing to skip the review a post you know needs help, but you don't have the time or energy or experience now to take that action.
When flagging or voting to delete a post, I always try to not destroy anything that is useful - so even if the answer starts with "I have the same issue and ..." and blathers on a bit and then has one clear answer / one clear tidbit or troubleshooting or something redeeming, it's far better to vote up/down based on how it relates to the other answers.
If other answers are worse than this attempt to help out and contribute, I'll often up vote an otherwise poor answer. If it's way worse than an existing answer, down vote and comment.
Only when it's totally un-editable would I delete it. Remember - even wrong answers are useful. By commenting and down voting, it shows others how an answer is wrong and helps them make their own progress on the same issue.
So TLDR - it's complicated, it's relative and it's often a matter of encouraging people to do better as opposed to judging an answer to some absolute standard without considering the entire post and the site's long term goals of amassing useful information that is open sourced, creative commons and collaboratively edited and curated and voted.