When I do that, it's just to keep all my leaders together to regroup them when I need to, my intention never is to have a bad leader as overall commander.
It´s really strange. I never put all my leaders in one province. When I play France or any other state with several leaders then I split them up to be able to use the abilities of every leader and not just one leader.
So we should be unable to keep dumb field marshalls from the battlefields?
You are not unable to keep dumb field marshalls from the battlefield.
First of all historical leaders are not dumb. Most of them are better than the generic 2/2/2 leaders. They might just not be as good as another historical leader.
Second you can already keep a leader from taking command from another leader: Don´t put both in the same battle/province/army.
No, but there are cases along the game's timeframe when power was given meritocratically. That's true for the Ottomans, Hungary under Corvinus, and I'm pretty sure it happened in Louis XIV's France and Friedrich II's Prussia.
Absolutely. In 400 years there certainly were more than a handful leaders who had risen to their place in the army/navy because of their abilities and not because their connections/sale of regiments/their status in the nobility/royal favour - but that does not mean that they would take command of the army from a higherranking other leader. They would serve under that leader and it would be the higherranking leaders orders and plan of battle the lowerranking leader would execute. Otherwise the higherranking leader would have probably the lowerranking leader executed for insubordination.
I'd rather suffer that than a defeat.
You would risk your rule as a monarch rather than deciding to not put all your leaders in 1 province? Or you would risk a revolt in the army rather than deciding beforehand to send the higherranking leader to another province? Just to be sure: You do play states that have more than 1 province? ;-)
Again, two things:
You can do that right now
Which means that it would be useless or at least a waste of limited time and resources (1.3 being the last patch announced) to add a complicated order of battle in which lowerranking leaders command part of the army but not the entire army to use the best ability score of all leaders present instead of the player deciding to send leader A to province X and leader B to province Y instead of sending BOTH to the same province and then wondering that the highest ranking leader takes command.
This change would give you a reason not to give a brigadier an army.
I see the titles more as generic terms as not only those titles changed meaning over the decades but different countries used them differently too.
There are only 5 different ranks (6 if you count monarchs extra that are above others even if they have the same rank) and "brigadier" I understand more like the "brigadier-general" used by the british army until 1922 and who could command an army unit or the "Brigadegeneral" as the lowest general rank:
https://en.wikipedia.org/wiki/Brigadier
However what and how many units you place under the command of a specific leader is your choice as a player alone.