It has been claimed that using a powered grinder actually generates heat which partially roasts the spices as well, while M&P is a slower and less heating process. Some cooks advise against using grinders for this reason but I'm not convinced there is a problem at all.
I have 2 M&Ps. A little one, about cup size, which is great for pepper, salt, small amounts of seed etc, and a larger one that weighs about 15kg and gives me a real workout to use. Cinnamon / Cassia Bark and Cardamom husks are a lot of work manually but the worst is dried ginger which is close to a rock.
I have to admit I've become lazy since purchasing a $20 grinder / blender kit.