Every doctor prescribes from his own experience. Risperdal has certain risks assosicated with it, so you should discuss those risks thoroughly, and consider other mood stabilizing adjuncts as well as alternatives, or just increasing the dose of Zoloft.
Many believe there are grounds for banning risperdal from the market. This is a blacklisted drug associated with numerous fatalities. There are other far more innocuous drugs available to do the job. In the United States there is a class action suit going on for those suffering from injury due to risperdal. Go to the PDR and read carefully about the side effects of Risperdal. Do a net/medline search. Come to your own conclusions.