If you dont want to teach you shouldnt have to. There are positives - money, being involved in the department (where I am, seems P/T teachers have higher status than those who are 'just' students), looks good on CV. I did 2 hr per week actual teaching (seminars) plus marking, attending lectures etc, total about 6hrs/wk. But my impression is it is becoming more difficult for PhD students to get teaching work - financial pressures mean Unis dont want to spend on this and are putting more of the load onto faculty.