Hmm, just thinking this can be a very useful exploit. If you don't have any civics or wonders that boost your experience (i.e. Theocracy/West Point) and you just want to make the best use of the one promotion your barracks will give you to attack a city (and you have a little money to spare) build a maceman, give him city raider and upgrade him to a grenadier! This will give you an extra 10% above what you would have got with Combat I.
Another example:
You've got to go and fight a Rifleman defending a city. You can build a unit that will get 3 promotions. The best attacking unit available to you is also Rifleman
Option 1: Build Rifleman and take Combat I, II and Pinch = bonuses of +45%*
Option 2: Build Maceman and take City Raider I, II and III = bonuses of +85%!! (note the extra +10% vs gunpowder for City Raider III). Then upgrade to a Rifleman
Going for Option 2 gives you a relative strength of 25.9 vs 21.75. It all depends on whether or not you want to spend 145 gold for the extra strenght (note though you are also saving 40 production)
I presume you can also use this to give gunpowder units first strike abilities if they were upgraded from archers.
(* from what I gather from the "combat explained" thread bonuses don't apply quite like this but I will explain it this way for simplicity purposes)