why would you even want to do something like that i mean wouldn't it make the game extremely frustrating
Because I get smoother movement (the mouse is very jerky for me, for some reason), because the transitions from walk-run-walk are smoother too, because I can trot my horse, because I can enjoy all the benefits my PC offers while laying lazily on my couch in front of my TV.

This is why I use it.
Im just curious, especially since with a keyboard, you can only walk and run (and by almost breaking your fingers, also sprint). With a gamepad, you'd have everything in between.
However, I'm still sceptical how aiming is going to get close to using a mouse but I really start to dislike using the keyboard for walking.
Yes, you do get all the movements in-between. You can walk really slow or really fast, like some NPC's do and you have to run-walk-run to keep up with them. This works for the horse too, you can walk him really slow, trot him (not available on keyboard unless using mods) canter slower or faster (not talking about sprinting), etc.
You get used to the aiming, trust me. Sure it will never be as accurate as a mouse (for me at least) but, hey, I svck at aiming and I still managed to play an Archer easily.