What if it were glued on? The idea is cool, but there's no way I would want something surgically implanted like that. It's just better to be able to remove it when needed.
I'd say a large part of the difference is mental. With an implant, your body internalizes the sensations, they're part of your body, your "one"-ness. With external sources, you may still feel the sensations, but they're not yours. Your body doesn't feel the fields, it feels an object that feels the fields.
It's hard to explain, and that's the best I got.