From the Student Teacher's Angle...
...DON'T accept IF...
you''re feeling burnt out/frustrated (or think you will be by the time the ST would be there); you're a fairly new teacher, still learning the ropes yourself; you have a difficult time giving constructive criticism without offending; you think you might frequently butt in during the STs lessons, and correct her right in front of the students...
I could go on, but you get my drift. The point is, please do an honest and thorough SELF evaluation first. I say this as a first year teacher, having completed a semester of student teaching last year, under a new teacher who fit the above description. It was pure misery.
When I completed a different semester of ST under a seasoned teacher who was the exact opposite, it was WONDERFUL experience for us both! Like the other posters said, she benefited from my extra pair of hands--also, at times, she was able to have me do things that she didn't necessarily enjoy, but which I was eager to do, because it was all a learning experience for me.
But, again, the self eval. on your part is critical, because you're the one who will set the tone.
Best of luck to you.