The argument was that scout is playable more often than some other cards. It's understood that it adds little to decks. Should it be bought more often than transmute, harvest or coppersmith? I think so.
You'll find that pretty much every high level player will disagree with this statement. Well, maybe not about transmute, but the others for sure. WanderingWinder gave a good quick explanation on why this is true in his post. But to discuss the points you made, basically most uses you give scout could be fulfilled by ANY action that gives +1 action. And the fact is, the other cards will do additional beneficial things. So for scout to be useful in any of those situations it has to be literally the only card with +action. And if thats the case, there is most likely a better strategy using big money. As for scout/great hall/crossroads/etc, this is one of those things that seems like it should be good at first, but experience shows that it really just doesnt.
As for coppersmith and mandarin, well theres a lot of cards which are less often useful. Manadrin is an all around pretty average card. Maybe you should try buying it more? Coppersmith is bought less often, but can be quite strong when it is. And I guess harvest is sort of boring mechanically. It's not the weakest card, but I can see an argument for cutting it based on the blandness.
I assume you know that you are probably talking about the JOG that is the current #2 on the Goko leaderboard....
But on the other hand.... Scout more playable than Coppersmith??? Not in my book! I also go out of my way to use Harvest. But I lose a lot when I do, so, there's that....
Just trying to clarify my position one more time here.
The times when coppersmith is the correct buy strengthen deck much, much more on average than the times
when it is correct to buy scout. My argument is that it is correct to buy scout more often than coppersmith. Simplest situation: engine deck with no other options (useful cards out, concerned about 3 pile, or not enough money left over), and silver might slow you down. Do you buy scout or nothing? I said probably scout. Best argument I read against this was buying nothing is probably best. I could be wrong, but I still don't see it. If you draw a green card it's a delayed cantrip and about even. In the end game, it's probably better than a cantrip a lot of the time. There's also deck order changing which could help your current turn, and possible interactions with other cards.
So, there aren't all that many times where you don't want
anything that costs 4 or less. Yeah, it has to be better than silver, but it also has to be better than all the other cheap cards, too. So you are building an engine, so much that you don't want silver, but there aren't any other engine components you want that cost less than $5? I mean, said engines are fairly likely to not be good. But also, why don't you want silver? Because it doesn't draw anything. Scout needs to be at least draw-neutral to not similarly be a clog, which means that in this super-engine-y deck, you have >25% victory cards? Really? This just, like, never happens. And if you're worried about a 3 pile ending (and if there are a couple of other empty piles of cheap cards leading to the reason you don't have anything else to buy, you probably should be), you really ought to be better positioning yourself for said ending, which scout certainly doesn't accomplish. Seriously, if you're so tied up, nothing is definitely better than scout - you just aren't going to have that many victory cards to draw. There are some 'interactions with other cards', but the problem is that these other cards are most often competing, and the interaction is really marginal to try to jump through so many hoops for.
Coppersmith definitely has a lot more situations where it is useful: Basically any engine where you aren't trashing copper, have a spare action, and usually you want +buy. So... tactician-based engines, wharf-based, council room, margrave, apothecary, as well as [any village]+[any smithy variant]+[any card that gives +buy]. Well, okay, it isn't as good as that sounds, because there is usually something better to do - but there's a reasonable amount of the time where it produces money better than any card this side of bank, and that is something - and there isn't ALWAYS something better to do.