The primary reason to pursue the Pyramids is to unlock all the Government civics. Not only that, but to do so far, FAR earlier than happens normally. HR is the only civic in that category for a loooong time and I always find the path to Constitution to be a pain. The other 4 categories stagger the availability of new civics more regularly.
Representation is a very powerful civic if you can get it early in the game and run some specialists. The +3 happy is nice too, considering that you probably won't have that many cities yet, and you won't necessarily have a ton of units for the HR bonus. I think Rep is more valuable earlier in the game than later. Later on, you'll have more cities so the happiness benefit is to a smaller percentage of cities (and may not matter with trade resources). The +3 research for specialists, while nice, seems to have less impact over time.
If you don't intend to use Rep early then there's really no point. You get some GE points I guess. Or as someone else mentioned, you can run Police State for warring. I suppose you could run US but you probably can't afford it early, and why bother if you're already in Slavery?