The settings.xml is considered personal, so for that reason the username+password are stored in the (user-)settings.xml. So in general there’s no reason to pass them as argument. (btw, passwords can be stored encrypted here) The maven-deploy-plugin has no option to pass them via commandline. However, I’ve seen hacks like:
<username>${internal.repo.username}</username>
And now you can do -Dinternal.repo.username=someUser