Martin, if MF Pro had to call GetMarketProfitAndLoss to get P/L, on top of getting the list of bets, this would be yet more load of the traffic and would add to the number of requests that the software makes to BetFair.
So the fact that the program can calculate the P/L on its own, using the matched bets alone, means that it saves you traffic and allows you to spend free requests on something more useful.