Well any application of marksdwarves really in the long term scale requires an extensive bolt industry, which granted, is actually quite easy to manage providing the resources can be found.
Personally I'd say a dozen untrained marksdwarves aren't going to win any battles by themselves, as opposed to anywhere around a dozen to 20 trained marksdwarves. But I guess that's not really fair, considering how a dozen untrained recruits isn't any better either. But let's say, the entire fortress... Yes, yes that would end every siege conceivable, barring absolute ragnarok.
Hell, you could even outfit your marksdwarf militia with any individual melee weapon on top of the crossbow, and have the literal formation fighting... Just make sure the crossbow is assigned first.

I do this.  It's annoying to keep track of which squads are 'real' and which ones only exist so civilians will carry crossbows and bolts, but it works.  I'll usually build a decoy entrance (a courtyard, floodgates/bridges on either end, surrounded by fortified walkways that can be accessed from inside the fort) to trap sieges in for annihilation by the whole fort.  It's kind of like a picnic.

I've been doing this. Expect to keep the forests permanently clear-cut if you want enough bolts for your militia to have any skill at all. It's an immense, labor-consuming industry, so don't think of it as a short cut.

I've been doing this. Expect to keep the forests permanently clear-cut if you want enough bolts for your militia to have any skill at all. It's an immense, labor-consuming industry, so don't think of it as a short cut.

This is why I like hunters. For every animal they kill, I get a bunch of bone bolts. Usually, by the time I have 3 or 4 hunters, I don't need anymore wooden bolts.

I always keep a minimum supply of 2000 copper bolts for my 40 marksdwarfs.

Loyalty cascades when everybody has crossbows are quite interesting. Less job cancellation spam, more horrifying deathgasm.

You make that sound like a bad thing.

I've started doing this with my initial 7 dwarves.  And the first 1-2 migrant waves  Any dwarf who isn't a hunter / miner / wood cutter gets drafted into a militia.  So my first two squads (20 dwarves) are my militia.

Schedule: Inactive year-round, using the (x) command in the schedule menu.
Supplies: I only let them carry drink, not food.
Ammo: Hunters get assigned ammo types (500 wood and 500 bone bolts), militia gets wood bolts only to start.

Things you'll need to make:
- Crossbows for everyone
- 1500+ Bolts, wood bolts are good enough to start (each log makes a stack of 25)
- Quivers (from leather, or the fall caravan)
- Flasks / waterskins (somewhat optional)

Uniform: Set to go "over" clothing, with exact matching
(1) crossbow
(1) iron mail shirt
(1) iron cap
(1) iron chain leggings
(1) iron high/low boots
(1) iron buckler

At least, I think "over" works for those items.  I don't even make the iron stuff until my 2nd fall, it's enough that they're walking around with the crossbow, quiver and bolts.  I figure mail shirts, boots, gauntlets and bucklers will be first off the forges.  They'll take what they can get, when they get it.

It is indeed high amusement when a kobold thief gets spotted and pin-cushioned by all the haulers / farmers / masons / mechanics.

Even when their schedule is set to inactive, with "no scheduled order", you *will* see them train at archery targets.

.W..........#...
.WA########D#...
.W..........#...
.WA########D#...
.W..........#...
.WA########D#...
.W..........#...

"." open air, "A" archery target
"#" floor, "W" wall, "D" dwarf
In the above diagram, the archery targets are placed on a raised walkway with a wall running the length of the room.  Dwarves stand on the walkways and shoot right-to-left.  Any bolts that miss the target will hit the wall and drop to the Z-level below because it's open air.  That will let you recycle bolts early on when the dwarves are not that adept at shooting the target.  The longer you make the walkways (10 tiles is probably a good minimum) the better, because they'll miss more.