Soya formulas contain glucose syrup, which can damage your baby's teeth with prolonged contact (I'm guessing she doesn't have any teeth yet though?!)
Some parents also have concerns about the presence of phytoestrogens in soya formulas. Phytoestrogens are oestrogen-like substances that occur naturally in many plants, including soya. There is some concern that a large intake of these could have an adverse effect on babies' hormonal development. This is a controversial issue which is yet to be resolved and agreed upon by the experts. However, the Food Standards Agency advises that, until a full review of the evidence both supporting and opposing soya formula has been completed, soya formula should only be given to babies under 12 months in exceptional circumstances and should not be the first formula of choice.
(from babycentre.co.uk)