You are quite right in that this form of assessing risk is a very blunt instrument.
It is generally used because 99% of clients do not trade as you indicate but tend to trade directionally and in unconnected markets.
Whilst the likelihood of say the FTSE moving heavily in one direction and the DAX in another is low that does not mean that it could not happen.
Your example of going long of 8 in the FTSE and Short 5 in the DAX ia a classic example of a strategy that would not work with fixed stops. But bear in mind that clients can move stop as as they wish once the trade has been executed. It is either use this method of control or use a margining method (like IG) where you get myriad emails saying that your position is at risk but never an idea of exactly what point is the stop level. Plus they tend to dribble the position down from say 10 to 5 to 2 to 1 ... effectively wiping out your entire account.
In general you pays your money and takes your choice.
To have a margining policy as you suggest would just be too complex for the average client to understand and would open the provider up to claims from the FOS.
