I don't want to be forced to optimize any more than I don't want to be forced to not optimize. I just can't fathom the people on both sides of the table who do nothing more than shut down other people's fun in favor of their own.
I am in agreement with this - nobody should be forced into playing in a way they don't want to, and everyone at the table should be working together to have as much fun as possible.
I also agree that, while many of the folks that claim themselves to be optimizers are meaning that they are combat optimizers, the term optimization is broad enough to apply to anyone trying to realize the truest possible vision of their particular character concept (like how I viewed it as "optimal" for my use-whatever-is-on-hand fighter to take the two-weapon fighting style and dual wielder feat because those enabled the widest variety of weapons without not having the benefit of the fighting style apply)
And players really should not know how monsters really work and what their challenge is if they don't want to spoil their fun.
That entirely depends on the approach to the game. My group runs with full transparency - all the players, even those that have never cracked the Monster Manual, are provided relevant knowledge about the monsters their characters face - because our approach to the game is that it is a game first, so knowing how many HP are left on a particular creature isn't a breech of some kind of "4th wall" so much as it is the quickest way to convey the in-character perceivable information of how much fight that creature has left in it so the character, by way of the player, can make appropriately informed decisions.
Plus, as a person that sits behind the DM screen at a ratio of about 300 session to 1 spent as a player - I'd be really bummed out if my knowledge as a DM inherently spoiled my fun as a player, but I love playing when I get the chance, so that's obviously not the case.
Bolded for emphasis, I think this relies on an often illusionary player/DM divide.
Absolutely. At my table, the only divide between player and DM is that the DM gets the tie-breaking vote - otherwise we're all equals, and I think that works fantastically well compared to other approaches to the player/DM relationship that I have experienced.
They take a cleric who refuse to heal...
As others have mentioned, a cleric that doesn't heal can be a very strong contribution to the party. Assuming they are still spending spell slots and using their domain features, they could be providing buff spells or dealing damage, and in either case helping the party reduce amount of damage taken overall - and that's just as good, if not better, than providing healing.
...a strength based PC with 12 strength
In one of my campaigns I get to see a 12 strength melee-focused Cleric side-by-side with an 18 strength melee-focused Warlock - and I get to see the player of that melee-cleric land more hits than the warlock when in melee because a difference of 3 in their modifiers is completely overshadowed by the d20 roll. It just doesn't even matter in practice that the Cleric only has a 12 - fights aren't lasting noticeably longer than they would if he had an 18 or even a 20 strength.
To me a good DM doesn't punish his PCs or the players for not having a cleric in play but instead provides access to healing in some ways (potions, scrolls, magic items, NPC healer erc) or adjust encounters accordingly.
I find that especially true in 5th edition, since potions of healing are on the standard adventuring gear list at a very affordable price - so them
not being easily available to the party is because the DM changed the default assumed availability for that to be so.
Even when I have a party with a cleric (or other healer) in play, I still have other healing options available - because I think "the cleric has to spend spell slots healing the party" is a terrible expectation to have, whether you are playing the cleric or just in a party with one.
You do not need a dedicated healer but you do need some amount of healing IMHO.
Between potions, hit dice, and a long rest, that's healing pretty much covered unless you are intentionally turning up the difficulty of each encounter beyond the default assumptions of the game.
If you are doing sandbox or a themed game you can get away with no healer at all. If you are doing an AP though or other prepublished adventure you will likely need some.
There is no inherent difference in those styles of game - you can get away with no healer at all in either. I know, because my group has done just that.
Party will be fine.
