If you allow both multipliers and keep the 1 to 1 conversion factor then at the end of the game a city (without national wonder bonuses) could get a 1 hammer to 4 gold/science conversion (factor 2 for production, factor 2 for gold/science) (1 hammer to 9 gold/science with national wonders). This is clearly far too powerful as you can buy back those hammers for 3 gold with the universal suffrage civic. If you lower the 1 hammer to 1 commerce conversion factor then building science/gold becomes extremely weak at the start of the game, even more useless as it is at the moment. It's just a bad idea to allow multiple multipliers that grow throughout the game to apply to a single economic element of the game. It's a typical source of unbalance in games. It means quadratic growth of an economic modifier which is an unstable factor in a game where all other economic modifiers are linear. In one of the earlier versions of civ4, there was such an element of multiple multipliers applying to a single economic element of the game namely the rush buy feature. You could get the multipliers for gold to apply to the commerce you got from cottages (and other sources) and you got the multipliers for production to apply to the hammers that were bought with this gold. This meant that gold rushing with developed cottages was way more powerful than any other method of production. In one of the earlier patches, the multipliers for production were removed from hammers that were bought with gold (gold rushing).