IMO, the purpose of the softphone feature is to allow someone to keep their ATA registered 'at home' while simultaneously (but occasionally) registering another device (or softphone) at the same time, for example, while traveling.

That said, you'd probably be OK from a Voipo standpoint if you kept an ATA registered at work (for personal use--not business use). Of course, there's the issue of using your employer's resources (bandwidth, electricity would be minimal but real), violations of employer's network usage policies, etc, that you'd have to watch out for. Obviously, if your usage patterns involve numerous simultaneous calls and it looks like you're abusing the privilege of having a 2nd device registered, your account will be flagged.