When inputting text on the last text field in a form on an iPhone and Android app, the enter key can be highlighted or (on iOS anyway) renamed "done", so that tapping it will submit the form. Therefore a "submit" button is not strictly necessary.
Not including the "submit" button means less visual clutter. On the other hand, perhaps some people sometimes don't tap the enter key after they finish inputting the final text.
Do you think it's good practice to always include a submit button? Do you think it makes a difference whether we're talking about an Android or iPhone app?