I sorta wondered if the writers/producers realized that they needed another disagreeable female but one capable of being a butt-kicker if the need arose. They had set Shannon in this character personality but unless the Others, or the monster, or ??? was subject to being nagged and griped to death, she wasn't what would be needed. Although the basic plot may have been thought out long before the individual eps were written, I suspect character introduction, and demise in some cases, probably isn't planned that far ahead.

__________________
Things fall apart, the center cannot hold;
Mere anarchy is loosed upon the world,
The blood-dimmed tide is loosed, and everywhere
The ceremony of innocence is drowned;

--Lines 2-6, "The Second Coming" by Wm. Butler Yeats