I concur it is generally useless, but there is a rare play where you can annex a city with the religion and faith purchase something. I did it once, when I didn't have Glory of God and my GP were already run up in price. Having the unit where I needed it when I needed it was worth it (in this case, I think I bought a Berber cav and I had a nearby city knocked down to black but no melee units to take it).