Spin stabs are possible because people who made this game either had no time to implement intertia or were just lazy. Thefore, human movement is just like in every other game, unrealistic. It doesn't fit well with many other aspects of this game which are very realistic compared to other video games.
I do understand that spin stab is "skilled" move or trick but it's simply not realistic. It's like something from Quake 3 Arena or Tribes. It takes skill to perfect but you'll never see any fighter irl fighting like that.
This game is built to be realistic medieval battle simulator. They couldn't make everything they wanted to so we have to live with certain badly done mechanics. Cmp is trying to fix that, nothing more.
Because blocking my great sword with a wooden stick is realistic right? or taking damage from a sword in plate? Or only having 4 attack options? I can go on and on, but my point is, this game might be realistic with the type of armor/weapons, but when it comes to game mechanics and balancing, realism needs to be thrown out the window. Spin stab, as canary said, is a technique that players learn to give them more attack options. An easy one to counter as well, simply block down till you hear the blocking sound, and you will weapon stun the person stabbing, probably killing him. If you want to look at it from a realism point of view (which you shouldn't) you wouldn't only have 4 directions in which to attack, you wouldnt stab the exact same way everytime you wanted to stab. Spinstabs offered another way to attack someone. This is something that doesn't need to be fixed, (except maybe on the jumping longspears) but all cmp has done is make stabs and overheads nearly useless, further dumbing down the combat.
Also, on a side note, in the beta, spin stab was there and was a recognized mechanic. Players complained about it then, and the devs left it in.
