So, I just did an experiment. I did a fresh embark with "Play Now!", dug out a single room, built a barracks via an armor stand, drafted all seven starting dwarves, and ordered them to train forever, planning to make no civic developments whatsoever and waiting for them to all starve to death. Everyone's in a squad, the squad's alert is set to train, the squad is assigned to the barracks.
Nobody is activated. One dwarf, in civilian mode, goes to the barracks and does individual drills incessantly. He never stops except to eat and drink. Of the remaining six dwarves, one is usually fishing, and the other five mill around the meeting area with no job. I eventually activate fishing and engraving on the one guy training and order a section of the wall be engraved as an experiment. He keeps training, never stopping to do the work. This state of affairs continues all through spring.
Summer starts, and things keep going the same way. For some reason, I build a second Armor Stand and create another barracks in the same room, so they're both overlapping. A little while after that, people suddenly start getting activated, starting with the one who was actually training. He starts organizing a dodging lesson. At one point I have four or five activated dwarves waiting to start the lesson. It never starts. This goes on for a long time. Eventually, people get bored/hungry/thirsty, deactivate themselves, and go eat. Sometimes they come back, though they aren't always activated when they do. I eventually reach a point where there are always exactly three activated dwarves: when one gives up, another one is instantly activated. Sometimes a non-activated dwarf has the "go to dodging demonstration" job, but they actually just mill around the meeting area. Three migrants come and I put them in the squad immediately, maxing it at 10. That doesn't really seem to change anything, though. Eventually the "three at a time" rule breaks and I have only two dwarves activated (though the one trying to lead the excercise is still there in his civilian clothes). Eventually, the one competent wrestler is the only activated dwarf, and the only one in the barracks, although about four others are claiming that they're on their way (but of course really aren't).
Autumn starts. We're now out of booze, and the only food is the raw fish that the two fisherdwarves keep bringing in (the original one, and one of last season's migrants). Good ol' Rakust (that's the one who started out actually training and keeps trying to organize a dodging lesson) finally throws in the towel and reverts to civilian clothes. He keeps waiting for training, though. There are now zero activated dwarves again. Everyone apart from Rakust stands at the meeting area with either "no job" or "go to combat training". Eight migrants arrive. I triple the size of the barracks room and put in about six more armor stands. I go to put the eight additional immigrants into their own squad, but I screw it up and muck up the original squad (when you create a new squad, people already in other squads still appear on the list, and disappear from their original squad when you put them in a new one). So I have to re-assign people to both squads. I turn all the new armor stands into barracks and assign each to either or both squads.
Rakust, having had his squad temporarily disrupted by my screw-up, is now back to individual combat drilling. All eight members of the new squad activate, along with maybe two members of the original squad. Feb in squad 1 and Thob in squad 2 both attempt to organize combat training. Squad 2 in its entirety shows up and start waiting with their leader. The vast majority of Squad 1 has no job, not even pretending to show up anymore. Neither drill starts. A caravan arrives, but I have no depot. A few people gain the "pickup equipment" job, in spite of the fortress not owning anything even remotely resembling military equipment. In group 1, Feb's dodging session actually begins! Clock it: 19th of Timber, late Autumn. Only one person is watching, with two others in his squad individually drilling and everyone else doing absolutely nothing. Squad 2 continues to wait for their drill to start.
I go to the scheduling menu, thinking that maybe Group 2 is still set to 10 minimum necessary for training. It's set to 2 for both groups, though (apparently it carries over to everyone using it). I up it to 5 anyway. Group 2 spontaneously begins training. A few people in Group 1 are waiting for combat training, even though the only lessons being led are for dodging. More than half of group 1 still has No Job. Group 2 suddenly goes back to waiting to train instead of actually doing it.
Winter begins. Group 2 starts up again. A bunch of people in Squad 1 spontaneously revert to civilians, and several of the people who have been civilians for months spontaneously activate to replace them. Group 1 is still mostly "waiting for combat training" with nobody trying to lead it. Group 2 is still at dodging practice, though half of them are asleep. The active/inactive positions in group 1 keep shuffling, but the active soldiers consistently fail to do anything. Driven by starvation, a few dwarves apparently start eating food directly out of the inventories of traders who are sitting at the border. Most of Squad 2 reverts to civilians to hunt rats to eat. People are activating and deactivating with higher and higher frequencies. Everyone remaining in the military is waiting for training. Rakust has once again taken to individual drilling in his civilian clothes. A few people start dodging practice again. Somebody goes start-raving mad, but doesn't do anything except start exhibiting "be going to dodging demonstration for eternity" syndrome again.
By spring we had pretty much reached the death-and-insanity stage due to food supplies having been empty for months, so I left it there.
I was hoping to learn something useful from this, but honestly the brokenness didn't even seem internally consistent. Things seemed to change in response to completely random actions on my part, or maybe the correlation was just coincidental. The only thing I can say for sure is that the second squad did a much much better job than the first when it came to actually joining the military and training. Having your starting militia commander or whatever he's called in a squad might hurt it somehow. Maybe I'll do some more testing later if I have time, or somebody else can give it a shot.