The bid price indicated on a live forex market [there are many - no universal source of data compared to a stock listed on a trading exchange] shows you the trade value available to you right now. A midpoint between bid and ask, or a 'most recent traded' rate [hard to quantify since there is no single exchange, but you could theoretically show the most recent traded rate within a given data set] shows you a theoretical value, not a value you could actually act on.
You say that a stock's price is typically measured by its last traded value - but that is in the context of representing the value of that currency in a general sense. If you were to actually sell that stock, you would sell it for the current bid price; if you were to actually buy that stock, you would pay the current ask price.
When calculating how to get from USD:GBP:CAD, you would need to use the actual ask price [or, from the other side, the bid price]. For other purposes, such as accounting for the value of a forex asset, sure you could use a midpoint.