I'd agree with early Great Scientist, but you're only realistically going to get that with Babylon. I've yet to try Holy Sites with Piety, as the prophets are generally better used to spread religion when you go Piety, but if you do happen to have a fairly strong base of followers, this seems like a good strategy.
I'd say that depends on what you're doing with religion - in terms of the raw benefits of from founder beliefs of spreading the faith, I think the fully enhanced holy site eclipses religion spread.
You have one belief that nets you 2 gold per city with your religion. The Holy Site grants 3 gold that can further be enhanced by modifiers within a city (and Piety adds another 25% with temples). So at their base, 2 prophets are worth 3 cities of gold. 3 cities that someone can't come along with their own prophets, missionaries, or inquisitors and burn away your well spent faith.
Then there's 1 culture per 5 followers. This faces a similar issue; due to city-pop sizes, as well as the likely-hood that multiple religions will be in one city, thus hogging followers - you'd probably be averaging about 2-3 culture per city. You're likely get a better value here than for gold since it's follower based, but again - this is all taking effort on your part. The holy site net's 3 culture. An astonishing 5 if you're able to pass the WC resolution. Again, this culture flow that can't be disrupted by your enemies faith-machine. Further, it too can be enhanced by modifiers and even converted to tourism via hotel etc.
As for Faith? 6 faith is equal to 3 cities with Pilgrimage. Plus, after your 4th or 5th Holy Site you should be looking at anywhere from 75-100 faith per turn even with a small empire; more than enough to spam missionaries if you're concerned with spreading the faith. This is assuming you also have a faith generating building. I almost always get pagodas, if I am able.
To cap it all off? This tile improvement has all THREE of the effects of those founder beliefs rolled into one spot that can't be targeted by your opponent's own religious agenda.