Imagine the case where you have purchased goods (one for 300 GBP and one for 250 EUR) for a client and you are invoicing him/her in EUR. Currently, QF will assume a 1:1 exchange rate meaning that the total of the invoice will be 550€ instead of 588.22€.
I sometimes have this issue as well with clients - currently I work around it by converting manually into a single currency, but I can see the potential value of it.