[@JaceBeleren]
*clears throat with just a little too much hacking*

first off, dark souls has proportions just a bit wrong with them being just a bit too thick and long but that's not too much of a problem. One thing considered in any type of sparring be it fencing or unarmed is reach. A zweihander not only has considerable reach like a spear, but it is useful throughout (where as the shaft of the spear can at best be used as a staff which despite the shaolin monk meme was not actually a very good one) because it is a long blade. Now, many vidya have two handed swords as weapons that pierce armour simply because it is two handed. This is just stupid. The blade would definitely cleave apart mail, gambesons and maybe even lamellar armours of the east but never, ever would one slice through plate. In theory, one can stab through plate with a two handed sword, however the conditions have to be very much in your favour because you need to get your hands in a funny position from which your fatfuck sword is hard to move from whilst easy to parry. Instead one would like with a longsword try to either wedge it into the weakpoints of one's plate armour (armpits and such) or flip it over to use the crossguard almost like a pick to bash through armour. With a two handed sword however, one also can in theory used the weight of the blade to create new weak points. If using enough force in a slashing motion the weapon can whack off a visor on a helmet to make the face oh so vulnerable, or to remove the rosette thingies protecting armpits (no clue what they're called in English). HOWEVER, the zweihander is oddly placed in dark souls because it was actually used in the very late medieval/proto renaissance period where full plate was rarely if ever used and one could comfortably hack off arms or legs or even heads which would be more or less exposed; apart from a breastplate/cuirass and a helmet covering the top of the head/face one would not wear much armour as bullets by now made armour have to be really thick to have any sort of protection and was very heavy thus making soldiers not have enough many nor energy to carry more than that. In this time period where plate was even more thick you would certainly not try to stab through it. 

You would not be worried about hitting your allies because you can simply hold onto the ricasso, or the small bit of unsharpened blade above the crossguard if the enemy or you have closed the distance effectively turning your weapon into a longsword. 

They were most certainly used commonly by people and not just a weapon of elite forces, I would say the proportion of swordsmen to handgunners and pikemen or halberdiers would be 1/3 or so. The long/warknife or messer which is essentially a one sided blade zweihander was actually one of the most popular weapons of peasant revolts and insurrections, although some historians contest that this was simply because parts of Europe had a law that said a one sided blade was a knife and thus legal, allowing many types of swords to be made for preparation for these things. 




Also, shit I have experience in:

Modern military equipment (not as much, only because I served [then deserted] and less than two years) 
Any close quarters combat from ancient history to WW1 trench fighting (done HEMA, Fencing and took history in school)