If you grab two magnets, and move opposing poles towards each other, you'll quickly notice that they not only repel one another perpendicularly, but there is a lateral force as well. This is due to the shape magnetic fields take around an object.
This is good in mod switches, as it naturally pushes the firing button shaft laterally against the bore it rides in. This helps make the electrical contact that allows current to flow from the mod tube, to the switch housing, and up through the switch shaft to the battery.
With a spring, the resistance is totally linear and perpendicular, allowing the firing button shaft to possibly remain centered in it's bore. When this happens, current instead travels through the spring itself. In high-wattage configurations, this can cause the spring to heat up. Heating a spring weakens it, causing reduced performance over time, or in situations where the firing button may catch on a burr ("crunchy" buttons), the spring could fail to release the button, causing an auto-firing situation until the button is pulled apart.
Also, magnets are powered by witchcraft.