Psychology
Having your fellow soldiers die next to you on the battlefield is already pretty traumatic, but at least it's not too hard to convince yourself they're at peace now, and often to "help them along their way" by burying them or having some other ceremony to dispose of the body (I've seen quite a few examples in fiction where one side asks or both sides agree to be allowed to bury their dead, which is a whole lot less practical here).
Now imagine they were to instead be frozen in place while screaming in pain. Are they dead or are they somehow still alive in there, forced to feel that pain and unable to move as long as the statue stands? If they're still alive, is there a way to reverse the "goldification"? Also, imagine the conflicts if one person thinks their thoughts are frozen and they can be brought back somehow, but another thinks they are in a constant state of unbearable pain and leaving them is just prolonging their suffering.
What might be even worse would be having to go back to the battlefield tomorrow and seeing them exactly how they died and unable to do anything about it.
Not only that, but the battlefield might still be scattered with the dead of the last army who tried to attack you.
Additionally, any given soldier getting hit on the battlefield could run around screaming, crying and begging while they slowly turn to gold.
This could very well destroy the morale of any enemies you might have.
More evasion
If you turn anything into gold with one touch, that would likely change the battle to focus more on avoiding getting hit.
Soldiers would prefer light or even no armour.
The enemy might focus more on sneaky tactics instead of facing you on an open battlefield.
A few arrows would destroy any tight formation (since it would be kind of difficult to hold a formation around some giant blocks of gold), so the enemy would be more thinly spread (either initially or eventually).
Any sort of constructs (catapult, etc.) would get disabled in one touch, so those would be virtually useless.
Some of those things might only be good for you if you know how to take advantage of it (assuming the enemy is at least somewhat well-prepared).
Although it might create more havoc, confusion and demotivation to mix Midas and regular weapons - focus the Midas weapons on what regular weapons are typically weak against.
Traps
Would the weapon turn liquid (e.g. water) into gold? If yes:
Take some enemies charging through ankle-high water.
Add one arrow.
Get a bunch of enemies who are stuck in place.
Although this would be more of a one-off trap.
Or launch a giant ball of something very light over your enemies - shoot an arrow at it while it's in mid-air to turn it into a giant ball of death.
5How much of an injury do they need? They need to inflict enough injury to actually kill outright normally, or just a flesh wound? Can they be used on people who are already dead? – Cadence – 2018-07-29T05:06:06.870
1Works only with deathblows and yes can turn corpses into gold. – Seraph Myrmidon – 2018-07-29T05:07:58.020
6it would certainly change melee, you would be effectively building a wall every few minutes. – John – 2018-07-29T05:08:33.347
65"Free money": more gold mean inflation, lot of new gold mean hyperinflation, and collapse of gold value. – Kepotx – 2018-07-29T05:13:58.427
3how common are these swords if everyone has one then both the gold and psychological gain would lose most of its value (and is magic common as well). but if its just the one sword then it could be a very worthwhile asset in war time (as stated bye everyone else). in peace time it could be used as a interesting way to execute someone (or a ritual killing for your gods) until we know more about your world it will be hard to say – Creed Arcon – 2018-07-29T06:19:44.983
1How much gold per corpse? What's the weapon itself made of? Honestly speaking, if I heard that the enemy army had that kind of weapon, I'd be more interested in stealing them than engaging in melee combat. So, lots and lots of archers and thieves in the night. – nzaman – 2018-07-29T11:39:22.890
As an added plot twist, there's no reason it has to be real usable gold... the troll that got turned into "gold" in The Tenth Kingdom comes to mind. – Michael – 2018-07-29T15:04:57.917
11How does conservation of mass work? Gold is about 19 g/cc people are around water = 1g/cc. A 150 pound warrior turns into a 3000 pound statue? Or turns into a 150 miniature warrior statue? Does it only turn the person into gold, or does it turn clothing etc? What stops the conversion from affecting the whole planet by proximity. – Sherwood Botsford – 2018-07-29T16:59:00.167
1One of the few uses I can think off is nice and heavy stuff to lob at your opponents during a siege regardless of being defenders or attackers. Perhaps the midas sword makes it harder for opponents to reach you as they trip over blood-slicked gold in weird positions. But otherwise it doesnt seem to have too many uses. Perhaps with an anti-midas sword to turn it back in order to control when and where corpses get deteriorating... – Demigan – 2018-07-29T16:59:40.873
1I'd be more worried about radiation that might be produced during the transmuting process. It's one thing to do strange things to your enemies bodies, but quite another thing to accidentally kill yourself and your army. I remember a story I read, "Three Hearts and Three lions", which such a transmutation (Troll into stone) explained why taking its gold was unlucky. – NomadMaker – 2018-07-30T01:12:17.283
1@Michael explain or link? not everyone has seen that. – Baldrickk – 2018-07-30T10:40:41.010
2just imagine the poor peasant who have to destone his field after a batle :-( – Madlozoz – 2018-07-30T12:29:26.077
2I wonder if it'd be useful for sinking smaller boats, collapsing drawbridges/fortifications/etc by hitting people on them with Midas arrows. – Vanguard3000 – 2018-07-31T17:29:24.353