A picture from a manual is not an evidence of anything beyond some artist painting it. I can show you a picture of a girl dressed as Easter bunny. Shall we conclude that it is custom in our age to dress up as Easter bunny? Perhaps we can even argue that Easter bunnies are real... Because there is a picture of one. There are classical paintings of the Jesus Christ surrounded by people in 16th century plate armor with Roman banners on background... Do I need to continue?
Fencing schools, through out the ages, especially renaissance, taught all kind of stuff, just as now you can go and become a martial artist in a "Northern Giraffe Kicking a Frog" style. The small sword and a dagger, was a very common occurrence in 17th century. But not because it was some sort of superior technique, it was a matter of practicality. At this point of time, people in large cities where not fine with someone walking around in a battle dress, neither you where allowed to carry weapons, unless it was required by your job or you had a certain status. If you where from upper class, you could probably carry full blown rapier and people wouldn't care but it's simply impractical in day to day business. Nevertheless you need something to protect yourself and short sword and a dagger, with good practice seam to have been doing just fine. There are techniques which help you with control of multiple enemies, there are technique to deal with someone who has a much heavier or a longer weapon (who would guess that some people didn't respect laws, especially if they wanted to kill you). So you can protect yourself, in a civilian situation. Keep in mind that for many upper class people, this would be a limit of experience with warfare, for a simple reason that army structures, training and equipment was done completely differently compared to 200 years before that. You where not trained as a warrior from the age of 6 to join ranks of elite cavalry and army core, you are more likely to be a bureaucrat. The point is that people where not completely stupid and used what worked and didn't use what didn't work.
The other side is that people often forget that main purpose of tournaments was socializing and entertainment. What it means is that people could have staged fights, with all kind of weapons and weird rules. There where entertainers who's sole job was to demonstrate prowess with weapons, riding, acrobatics and etc. There where flight archery competitions, I've heard Ottomans where really good at it. The point is, what happened there, have very little relevance to the warfare.
And of course other oddities. Like there where a case of judge decision where sides had to solve their argument in duel, the weapon was.... fish. I can't find right now where I've seen that, but there was a depiction attached. A lot of argument where solved by duels and it's not rare that judge would decide on weapons. Which resulted in case of people dueling with rakes, chairs and etc.
Unfortunately we don't know many details about medieval societies. Especially when it comes to a nerdy subjects such as "where arrowheads at Agincourt case hardened or not?", half of the people there on actual battlefield probably would not know or care. But we do care and want to know
Mount & Blade is a game, but so far it managed to stay mostly in the realm of historical fiction. This attracts and detracts different groups of people, this has a heavy influence on what kind of mods are made. Which in return influences longivity of the game and brings hundreds if not thousands of hours of fun for the people who like it. In it's own way it's a niche game and it is benefitial for everyone involved if it stays like that.
The argument that dual wielding could be used in tournaments is a good one but then why not have all other things too, including goofy stuff, like nets, humongous helmets and fish fights? Or rather the same resources could go into extending other, more important things?
I personally would be much happier if instead of dual wielding, same resources would go into:
- war elephants
- chariots
- field war engines
- early firearms
- ability to pull rider from the horse
- flails
- deeper interaction with realm
- custom bows and armors
- various armor and weapon smiths who could make custom things for you or even teach you something new
- durability of armor (both in battle and long term)
- custom formations, so we could finally have a proper shot and pike mod
- regiment designer where you not only set certain ranges of equipment but decide on ratio of infantry/archer/cavalry in regiments
- a proper mercenary company campaign
All this could be in a realm of possibility and has much better grounding in history. Adding dual wielding would add a flavor to a part of the game where it already developed the most. We really need more content, not visual fluff.