As far as I know, religions spread naturally from city to city - Even if you aren't the founder w/ holy city
I am pretty sure all natural religious spread is via the holy city, your average city can't spread its religion without missionaries.
As to razing holy cities, if you already have a couple religions, and know you won't be spreading that one (or won't get the GP) then it makes sense, more likely to get your wanted religions to spread naturally.
Safix: it depends on the situation.
First: where is the city (if it is owned by some teching civ right on your border, go for it. if it is deep in a well defended empire, that may be a problem.
Second: who owns it. If it is Issy, you may be in for some fun. If it is owned by Toku, it probably won't matter.
Third: is the religion actually widespread or just like 1 city per civ (in which case you may be able to convert people via missionaries rather easily).
Fourth: How far into the game (if only Buddhism and Hinduism have been founded, then some time may solve all problems.
Rally there are two many variables, but one certainty: if you can easily get it at little cost, grab it since holy cities are cash cows, especially if someone spread it for you.