Okay. So there's this bug where drunk dwarves are too cautious to spar.
I've found part of a solution, provided your fort has access to goblin mercenaries.
Why? Apparently, goblin mercenaries never drink booze. None of my 10 goblin mercs has ever touched booze. Don't know if that's intentional, or a bug.
And they do spar. Once they get some individual training session done, and gained some skills, they will start sparring. And it will yield sparring reports in your alerts, so they're not just standing there.
They will spar with your dwarves too. Your dwarves will still be too cautious to attack, but at least they'll gain skills in dodging, shield, armor and / or weapon (parrying)
Be careful with taverns though. Goblins getting served drinks by barkeepers seem to have a very high mortality rate. Best not have any booze and mugs in your taverns, and no tavern keepers at all, until that bug with mugs being worn is fixed.
I'm not sure if they actually drink the booze served by tavern keepers though. Considering worn mugs are still holding the booze, and the fact that I've had no mystery deaths ever since I stopped using tavern keepers, it does look like it's not the alcohol, but the tavern keepers' bugged action of decorating the dwarf with a mug that's causing the mystery tavern deaths.
The problem with the goblins not drinking would be that over time, your goblins will be more and more distracted by lack of alcohol. It does not seem to be very severe. My oldest merc (been in my fort for nearly 3 years now) has a "distracted by being kept from alcohol". It's not red yet. By giving them enough downtime for other needs, it should not be problematic.