99% of the time, razing a holy city is a very stupid thing to do. No benefits for doing it.
Only exception is when the religion it represents hasn't spread at all + when the city will forever be a trash city (small, not productive, etc.) + when it's located at a place on the map that is quite far from your Empire (meaning it'll be VERY hard to defend it from rivals if you don't want to expand your Empire in this region).
Under these circumstances it might be useful to raze it: if you can't get the benefits from the Holy city (1 gold per city per turn) because it's too far from you, no one else should. BUT as long as you think you'll be able to hold on to the city, keep it. Lots of income.