Interestingly, that's the part I felt like you were ignoring when you described what I was suggesting as homebrewing. I had meant for you to invoke this! If you find a scenario that you feel doesn't make sense without help working - let it work!
Right, but as I said, I'd rather reverse this. I don't want the DM to give permission to let it work, I feel that is a problem because the more RAW and conservative DMs will then prevent logical actions from meaningfully helping. Instead, I'd rather the DM step in when it doesn't work. This gives the players free reign to immerse themselves in the story, and only when they begin breaking that immersion is the hand of the DM felt to keep them from breaking things in silly ways.
I'm not sure that's the intent - I think the intent is to make each roll more important. That's part of why (AFAICT) they've been pushing the "don't roll unless there is interesting consequences". I think they're trying to teach DMs to be both more generous to player input, and also to not let a bad roll derail the story that's being told.
This doesn't follow to me, and if this is their intent, they are making a big mistake I feel.
The more important each roll is, the more the players want to invest resources into making sure that roll succeeds. If WoTC is trying to make each roll more important, while also limiting the resources people can put to make those rolls succeed, then it is only going to drive players to more extreme lengths. Because these rolls matter so much, they cannot fail, and so they will seek to get ever more extreme bonuses.
Some people may think that will lead to more "creative play" but I know for me that I've often used creative solutions that grant advantage as the Help Action. That means if the Help Action is now strictly defined, then creative solutions will be constrained. "No, that can't grant advantage, because to grant advantage on a skill is the Help Action, and it requires you to be proficient". That is a thing that will be said.
I'm not sure there's danger in that happening. I can't imagine the designers ever intending to make the rules interfere with team building and teamwork. I mean, I guess it could happen with unintended consequences? I'd hope not.
But there is absolutely a danger of this happening. If you have a player about to make a roll, and they have already used Guidance that day, and no one else shares in their skill proficiency... that's it. That player must stand alone, and the rest of the party will check out, because there is literally nothing they can do or engage with.
I have had many groups struggle with team cohesion, because there is very little that you can actually do to help each other. In combat many of us act purely as individuals, because it is the most efficient way, and there is no such thing as a combo attack or anything else. Other than a caster setting a zone spell, there isn't much that say, two fighters can actually do to support each other. Now out-of-combat is going to end up the same way, there isn't much of anything we can do to support each other, so the most efficient way for us to progress is to just let each individual act on their own. Having a wing-man is only useful if you both are trained in the skill you know you will be using, if you don't share skills, you can't support each other.
If there is no mechanical incentive to work together, then you are relying solely on people to make up reasons to work together.
True. As an aside, Help is an action and Flanking is "free" (with positioning). Both are pretty much narratively the same thing. I approve of the Help action in combat, but I don't like Flanking (which I also find more fiddly rules-wise than its worth).
I find flanking can be fine. The only time it is less good is when the classes provide different mechanics to get advantage in combat. Flanking makes a Wolf Barbarian far far less powerful, for a common example. But, as I said, in games with flanking, I find the team works far harder at positioning with each other and double-teaming enemies. Meanwhile, in games without it, we can often find ourselves not paying any attention to our allies, because it doesn't affect us until they either win or lose.
I get where you're coming from now. You just don't want any barriers to players using teamwork to accomplish goals. It's an excellent motive.
Basically.
Fair. I think it's fine, but I guess I'd be fine to rule against it often, so maybe I'd rather it was changed. Another place where I'd probably rule against it is in situations where one skill might be complimentary to another skill (Like good-cop bad-cop in an interrogation. One could argue that one character's intimidation could give advantage to another's persuasion!)
Exactly, that is such a good use of skills and tropes, I want that to be something the players can decide to do, without them looking at the rule and deciding that they can't assist in the interrogation, because the Paladin is using Intimidation, and they don't have intimidation, so they aren't allowed to help.
Another thing to consider is that there is a chilling effect from rules like this. Even if a DM might make an exception for a rule, players are often trained to not suggest things that break the rules. If they know the rule says they must share proficiency in the skill, many of them won't suggest anything that would break that, because that's against the rules and therefore it won't work. Even if the DM might allow it to work.