I remember LoyceV saying that he uses Loyce mobile while on transit or so, that also shows that you can have the need to use the forum in the public place.
In LoyceV's case, I believe the user talked about people not contacting them on the moniker that's strictly for when they're on the move because it's not as secured as the main account. We aren't taking about security of account here but privacy.
I've said both: don't trust my Mobile, and I don't like that "anyone" can see my username in big font when I'm "in public".
I guess a much easier solution would be to use a privacy screen protector. The downside is that it destroys the viewing angle of your device.