What are the positives and negatives of using preventative antibiotics?

Prophylaxis: The positive effects are that they may prevent the onset of certain infections, but many forms of prophylaxis have been shown to be ineffective. Negatives include allergic reactions or inductions of allergy, changes in your normal bacterial population which results in severe secondary infections such as candidiasis, c. Difficile colitis, and so on and so on.

Preventing what?: The negatives: the usual side effects of antibiotics, including the potential for clostridium difficile infection, as well as the risk of developing antibiotic resistance. The positives: that would depend on what you were trying to prevent.