You repeatedly use very colorful language to say all this doesn't make sense, but you don't give us much indication what exactly you don't think makes sense about it. The spellcasting rules say that these spells have a somatic component and a material component, meaning you need to hold something in your hand and manipulate it in order to cast the spell. Just like the fighter needs to hold a sword in his hand and manipulate it in order to attack enemies. All of these facts you list are natural consequences of this requirement: you can't use a symbol on your armor or around your neck because you can't manipulate it, just like the fighter can't attack with a sword strapped to his back. You can't use a symbol that casts spells for one class to cast for another class, just like the fighter can't use a sword in his hand to shoot arrows.
Look, I know you said you don't want people defending the rules, that you're trying to fix what's broken, but I honestly can't get a good read on what you think is broken about this, and so I can't give you much in the way of advice on fixing it. What in-universe logic are you envisioning here that these bullet points contradict, and what do you think should be happening instead? If I try to figure that out by simply reversing the bullet points, I get results like "You should be able to cast using a material component pouch even if both your hands are occupied" and "You should be able to cast warlock spells with a holy symbol", but that doesn't seem right either. Do you perhaps just feel that cleric spells shouldn't have material and/or somatic components? Maybe try that?
See, this is different, because this actually doesn't make sense. A rules patch saying that a hand holding an appropriate focus can always be used to perform somatic components does seem to be in order.
Okay fair enough.
First off, my true desire is for it not to matter where you put your god's symbol. I see nothing good coming from "you can walk around with an amulet around your neck, but it will inconvenience you later on".
In the general sense, it shouldn't matter whether your display is on your shield, or on your forehead, or on your chest. It's much more important you get to describe a cool character, and details take a distant second place to that.
But this doesn't mean we should abandon the D&D:isms of the game.
A wizard should imho need to use a hand to cast his spells (whether by literal hand, or wand, or orb or whatever). A bard has traditionally been forced to create her magic by music - 5E's interpretation is a reasonable compromise between preserving this image and not actually forcing you to make noise (disregarding that your spell most likely has a verbal component).
That doesn't have to... that SHOULDN'T have to mean the rules for this needs to be byzanthine and perplexing, like they are in 5E (and probably AD&D and d20 too).
And they should sure as hell not allow stupid BS moves like the excrecable "drop, cast, pick up", much less make that an actual optimal strategy.
When I started this thread I was doing so with the firm belief we should be able to express the good stuff (the D&D-isms we want to preserve) without the bad stuff but to do so MUCH SIMPLER than the current rules.
5E is not nearly as complex and fiddly as previous editions. With object interaction and spell component fulfillment as easily ignorable but still glaring exceptions.
WHY, I ask. Why were the rules this muddy when they can be so much simpler and better (while saying essentially the same thing minus the headaches and the nerdrage)?
We should not have to even TALK about where your hand is, or which component goes where. "It's natural you need a hand if your spell has a material component" is a much to simulationist and detail-obsessed question to ask.
We should simply ask the players to describe what their character looks like when he or she does heroic stuff, and from that simply say what actions are restricted or outright impossible.
You fight with two axes? Way cool... but you can't also carry the lantern or cast any spells.
Fiddling about with "can't I shuck one of my axes and cast real quick and then draw the axe again" is micro-management. Within a single round that's a waste. Much better and more in the spirit of 5e to simply define what weapon/shield combos that are available for you if you want to cast a spell.
All assuming "that round". Nothing stops you from choosing "I hold a single axe" the next round and cast your spell then.
What says we must have fiddly rules to achieve complete "realism" for sub-round hand use combos in the first place?
I'm sure if we give up the ability to cast a spell despite dualwielding (or whatever) we gain SOO MUCH MORE in saved text on the page, and saved brain hurt trying to grok all those maddeningly fiddly rules figments, not to mention how they all end up being monumentally pointless
anyway since you can always drop-cast-pickup to completely evade them anyway...
That's it
