Using magnets for extra grip is very plausible if its done right. Ellis is only partly right about magnetic hold-down adding strain the the drive train; there will certainly be extra force on the gearbox bearings and adding outside axle bearings is always a good idea, magnetic hold-down or not. While the bot is driving in a straight line the only extra stress on the drive train is the extra rolling resistance of the wheels, which will not be much. Magnetic hold-down is not like adding extra weight to the bot as the bot's inertia stays the same and the effort needed to accelerate it will also be the same.

The extra strain comes when a 4 wheeled bot tries to turn with skid steering; the wheels won't break traction so easily and the motors will draw more current. This wouldn't apply to a two wheeled bot as its not skid steering. All bot designs would use more current in a pushing match as the wheels have more grip and need more current to break traction.

IMHO, adding around 5 to 10 Kg of magnetic hold down to a two wheeled bot would give it really useful traction while not increasing the current draw and drivetrain stress too much. The NIB magnets needed for that probably wouldn't be too large or expensive either; well worth some experimentation!