Are unlinked mesh islands of the same object considered bad modeling? And, when should I merge meshes and when should I keep them separate?
I'd say it depends. If you want to add meshes in edit mode, they'll be added to your current object. Sometimes this is what you want, sometimes not. A mesh added in edit mode will also share any materials and object data of the original object.
Also note that your Add options are different depending if you are in Object vs Edit mode.