There's a big difference between a grinder and a grain mill, a grinder grinds, a grain mill crushes through rollers and the results are completely different. Crushing your own grain it's important that you keep as much of the husk intact as possible as it helps to act as a filter bed in the mash tun, using a grinder can break the husk into smaller pieces so it's not ideal.
Having said that folk do use grinders as they tend to be a lot cheaper than a malt mill.