Of course any Shepard should care at least a little bit about Earth regardless of his or her own origin. That wasn't the issue (from my perspective). The issue was that BioWare didn't do enough to make me, the player, care about Earth. They just assumed "Oh, we live on Earth, so everyone will automatically be emotionally invested!" but that's never really worked for me. Even in Star Trek movies, for example, I kind of roll my eyes when the inevitable threat to Earth pops up. At this point it's become an overused trope, more cliched and boring than gripping and emotional.