The tape saturation included with MasterRig (and Cubase’s Quadrafuzz v2) is nice but it’s very blunt. It usually starts saturating and compressing heavily even when using the minimum settings, which can make it unsuitable (or difficult) to use in a mastering context.
The algorithm used by Magneto II is far more subtle (I assume it’s modeled after master tape?), so it would be nice to have it as an option for these cases where the old tape algorithm is too much.
(Note that the plugin version of Magneto has a bad bug that causes bass loss. This should be fixed before it’s ever added to Wavelab!)