Using unencrypted key and real key, benefits?

I am reading the docs for generating keys in an application. I'm not sure what effect using a simple String key has a real key. For example, when my users register, they must provide a unique username:

class User {
   /** Key type = unencoded string. */
   @PrimaryKey
   private String name;
}

      

        
        
        
      

    

now if I understand the docs correctly, I can still generate named keys and entity groups using this: ??

// Find an instance of this entity:
User user = pm.findObjectById(User.class, "myusername");

// Create a new obj and put it in same entity group:
Key key = new KeyFactory.Builder(
  User.class.getSimpleName(), "myusername")
    .addChild(Goat.class.getSimpleName(), "baa").getKey();
Goat goat = new Goat();
goat.setKey(key);
pm.makePersistent(goat);

      

        
        
        
      

    

the Goat instance should now be in the same entity group as this user, right? I mean there is no problem with leaving the primary key User as just a raw string?

Is there a performance advantage when using a key? Should I update to:

class User {
   /** Key type = unencoded string. */
   @PrimaryKey
   private Key key;
}

// Generate like:
Key key = KeyFactory.createKey(
    User.class.getSimpleName(), 
    "myusername");
user.setKey(key);

      

        
        
        
      

    

it's pretty much the same, i will still generate a key using a unique username anyway,

thanks