If you somehow get your major cities to share the same religion and the diplomatic situation allows it switching to state religion and a religious civic may well be worth it.
Most of the time you have to spread it actively, wasting production on missionaries and, if you didn't found it yourself, even giving the AI an extra gold with every one of them as well as the chance to spy on you.
Then again, without religion you can't get happiness from it, boost research with monasteries
You don't get the +1 for state religion (in cities where you have it) but you can still get a plus with temples. monasteries work independent from your state religion so it's neither a pro nor a con.
Also, since I almost always play pure SE now it's almost essential to have these things
What exactly is the influence of a state religion on a SE?
On the pro:
-You can have a civic that gives +25% building prod, +2 XP or +100% GP (in all cities with it)
-You can make better friends with your religious buddies
-You get a further +1 of happiness (in all cities with it)
-You can use Sangkore University and/or Spiral Minaret
On the con:
- Without being spiritual you need at least 2 turns of anarchy
- You may need quite some ressources to build monasteries and missionaries. (with org. rel. you don't need monasteries) if your religion did not spread to all your cities all by itself.
-You pay high maintenance costs for your religious civic
-You get negative modifiers with a lot of other civs eventually
-All non-statereligion cities lose their cultural bonus for having a religion, 1 for normal, 5 for founding cities.
I often switch to org. rel. or theo IF i have my cities with the same religion or can spare a city to spam missionaries. I almost never switch to a state religion early in the game.
And with free religion available i often take it as it is cheaper and opens up a lot of diplomatic options.
I would say it heavily depends on the situation you are in, i think in about 40-50% of my games i don't adopt a state religion.
I play on immortal and 99% of the time it is better to have a religion than not
your opinion obviously. But in other words: You are missing an important part of the game, mainly to be able to influence other civs diplomatically without being bound to a certain block of religion.