I think you should not join US army just for the sake of maintaining your immigration status. Serving in one's national army is a patriotic act by the highest standard. So, join the army if you will really feel proud of serving your country (i.e. US). And it is truth that, in modern and civilized nations, the national army is generally used to protect and promote national interest. If it were to promote an interest of a person/party/group, then you may not be able to say it is a real service to the nation, but generally it is no so.I think joining US army and subsequently becoming US citizen and feeling pride and proud of your country is differnt from 'Gurkhas' serving in British and Indian army.