Finally, and I'll make this post as an addendum because the first one is huge, and this is slightly off topic and there's no way around it...
Both the Supply Wagon and the Mystic Merchant feel like cheating to me.
Supply wagon thoughts - The supply wagon in particular, allows you to store an almost unlimited number of troops and prisoners, then pull them out of thin air whenever you need them. I got in the habit of zipping around the map with just my companions, and unpacking a tailor made warband for each encounter, with 300-400 troops stashed there. I also had something like ~120 dena pirate prisoners, that I would periodically take ten from and hire them.
I also would individually hire ship captains or caravan masters, who were well worth the -10 morale hit, and would eventually concede no matter how many times they refused you asking.
The supply wagon also somewhat ruins the game's economy, by allowing you to hoover up every battlefield, sell all your crap despite towns being out of scillingas or space for more items, and keep an infinite number of prisoners. Not to mention carry a more or less unlimited number of supplies. With the wagon, I could jump on a ship and land with 500 men and plenty of supplies in Ireland. I was able to amass more than a hundred thousand scillingas in savings after about a month of having the wagon. And I'd say that about 75,000 of it came from being able to sell everything without limits, and storing prisoners.
Without the wagon, I find the lair MUCH more useful as a place to store troops and food, and getting a castle or village near where you expand, or keeping a supply depot is very important. It's also important, with big armies, to forego carrying too many horses or spare weapons in your inventory, and stocking up with food if you intend to win a siege. You can't just zip to a location, then unpack your army once you get there. And again, it makes lairs, castle, village and household storage much more useful.
Yes. It is a little bit annoying to have to babysit clerics, caravan masters, whores and spies (lair helps with that, but caravan masters seem destined to death). Who I would ordinarily put in the supply wagon space. But even though the wagon has some good points, its just too abusable, and I guess I'm too weak to keep from abusing it. So I don't build it anymore. It just feels like the game's economy was intended to function by not letting you get full value from every item of loot and every prisoner. And at least now, it pays to invest in inventory management...which I think already is supposed to represent your 'supply train'.
This would work if the supply wagon was better at dumping all your troops and prisoners into your main party upon leaving the screen, but it doesn't currently work. And there's still the problem with selling all the battlefield detritus long past when most towns should be able to support it. There's also a fun bug where it will sporadically duplicate your prisoners, if you have too many of them and add them to your actual army as troops. That's how I got a surprise gift of 68 geogoth seaxa.
Mystic merchants thoughts - It's just outright cheating, and a lamentable inclusion. I no longer buy anything from the armorer or weaponsmith. Only from the mystic merchant, where I might get a tempered or balanced weapon at a fraction of the cost. I hate this guy, and I hate what he does to the game, but I can't stop using him. I wish I knew how to remove him, so that I'd actually feel tempted, someday, to spend money in the armorer or weaponsmith again.
If there is a way to remove the merchant from the game, without having to recompile everything in python, please let me know.