It's the front of house/back of house distinction. Front of house should use fixed point, back of house should use floating point. Unless you're doing trading, you want really strict rules with regards to rounding and such, which are going to be easier to achieve with fixed point.
I don't think it is that clear. The split I think is between calculating settlement amounts which lead to real transfers of money and so should be fixed point whilst risk, pricing (thus trading) and valuation use models which need many calculations so need to be floating point.
