Here's a practical example:
I flagged this answer before it was edited and contained just one sentence which didn't answer the question at all. So I flagged it as not an answer. Later, the answer was edited, and I saw that, but I couldn't find a way to take my flag back (which hadn't been reviewed by the time), and consequently I lost 10 flag weight because the moderator deemed my flag as invalid ( I really doubt whether he would deem it as such had the answer not been edited).
So, wouldn't it be good to enable taking the flag back? Or, alternatively, let the moderator judge the validness of the flag by the version of the post that was current by the time of the flag?