People that spam the site usually target specific pages, with the goal of putting in as many links as possible, sometimes anonymously. Limiting the amount of answers they can post means we stop pages being overrun.
But maybe there is a better way to deal with this issue. I myself have found that sometimes I want to go back and provide a second opinion on an answer I've already written - and have been shown the "you've already answered this question" message...
So I worked around it by adding a comment to my original answer explain my follow up thoughts.
Another option would be to edit my original answer to include the new information, I guess whichever is easier at the time makes most sense.
Does anyone else have thoughts or suggestions about Nickie's question? I'd love to hear them!