There is a ready-to-go mod for ore carts: Minecart
but, as to players building better mines with rails, I dont think it will happen. This is because to mine well, one needs to travel vertically. Rails are not built for vertical movement. it requires building a 3×3 mineshaft, with a spiral, and 2 rails for every vertical node downwards. In a game where steel is expensive and might be the very thing you are mining for, this does not make economic sense to do.
Now, it would be VERY different if ores spawned in large rare clusters, near the surface. Then, you would want to build a horizantal mine, and carts would make a lot of sense.
Still, I think that adding minecarts would be a nice way to have automated transportation of people and goods