You seemed to have missed the point here. The distance between model and severe terrain is almost never divided evenly, even in half-inches. (btw 0,5" IS annoying enough). At least when I do positioning, I want to do it in a way that benefits me and the model in question - in relation to enemy positioning. As said, gun/spell/melee ranges are static, so I very much care whether my Rusty Alyce is 12" away from the enemy or not.
This means I'm not actively seeking to place my models in even distances from severe terrain - rather I want to find a good position and the severe terrain just happens to be X" away. Now, if I need to move to severe terrain, I'm also often not trying to even out the inches to handle severe terrain better, but trying to find a good position.
Say a model with mv 5 is 3,4" away from severe terrain. You have 3,4" normal movement, when the base touches the terrain, then the rest is 0,8". That doesn't even get your base (approx 1,25" for 30mm) completely to terrain, so next action/activation you have similar problems if you want to move out.