So they made walking quiet (no sound) and running loud (pad pad pad..).
In JK, the concept of running is more like real life. Yes you have to hold a key to run (unless you enable it always on), because in real life you have to put forth more effort to run than to walk.
In JK/MOTS walking is for making precise movements, and moving slowly (you have to do that sometimes in SP). Running is for getting places fast.. and it builds up to a run (you go faster if you run longer.. not infinitely fast, it peaks out after a few seconds). When manuvering along cliffs, I try to go slow so I don't run off the edge.