I don't think there's any way I can convince you the bonus content is that - extra - instead of something cut, so I hope when more details come out about Civilization V you'll understand than you will have oodles to do and many civs to master even if you don't want to grab the edition with Babylon in it.
The line is extremely blurry in this particular case of a whole civilization being unavailable. Most deluxe content in other games are some sort of weapons that you will only need for a few levels to be cool and quickly become useless, extra vehicles that don't matter and such. A whole civilization cut out of the game is a no no. Yes, cut out. Music, behind the scene, figurines, extra weapons that don't last for long in the game, a bit of experience to get you boosted at the beginning, THAT is extra. A civilization that is unavalaible and can't input its personality, units, diplomacy in any of the (full length) games that I will be playing. THAT is cut out.