Why? The few more admin cap it gives don't do much as seen in the tables. However the fan. pacifist drawbacks on (war) diplomacy are severe. Can we actually win by score points as a fan. pacifist nowadays?
I mean outside of the fact that Fanatic Pacifism is the bestestest ethic (because I like it, and that is obviously the only reason necessary

), it also provides you with the admin cap when you want it most. Tech and Unity cost isn't all that high during the early game, but production values for them are at their lowest during the first 30 years or so, so any cost increase will hurt quite a bit. The cap is also pretty useful if you want to rush for costly techs (like Mega Engineering).
The drawback is there, but if you are playing SP, there is really no necessity to wage any wars at all. With Habitats and Ringworlds, you can easily outperform other AI empires. If you look at the dev clash (which, granted, is not cut-throat in terms of play style), I have been doing quite well there by befriending my neighbors. The fact that I didn't have to invest into military all that much allowed me to focus heavily on mega structures, and while I will lose my #1 spot to the Cyber-Equestrians in the near future, I will definitely remain among the galactic super powers for the rest of the game.