I found out that the reason Joss killed Tara was because he had an ambition.
As a writer/director/producer/JERK/whatever he wanted to kill of a MAJOR Character on a show!
It was not to advance the plot, boost ratings, or any SANE reason...
There are many reasons I call him Joss the Jerk in my reviews.

Magic addicted? Really? WHY WASN'T WILLOW SENT TO THE COVEN RIGHT AFTER SHE RESTORED ANGELS SOUL!

The answer is magic is not addictive, Joss is a SADISTIC BASTARD!

Not the only story where Willow dies instead of Tara, but I think it is the only where both are alive at the end.

Sorry I don't know the titles of the others.