Why are Q Magnets in a plastic casing?

The plastic casing is to reduce instances of any allergies to metals used in the Q Magnets. Our smaller Q Magnets which don't have plastic casing are gold plated. 

Some people are allergic to gold plating, so the larger models are encased in non-allergic plastic casing, also the stickers/graphics are preserved this way.

With neodymium magnets, as soon as the silver or gold plating is cracked, they will begin to rust and it won't be too long before they lose their effect.
So the plastic case also protects the magnet and provides longer shelf-life. 

In terms of the magnetic field, unless a material contains ferromagnetic metals such as iron, nickel or cobalt, then the magnetic field will mostly pass through it uninterrupted, the same as for instance water, skin, wood, glass, aluminum or bone. The only slight difference the plastic casing will make is to separate the magnet from the skin by about 1mm. This effect would be negligible, but the benefits of having the plastic casing completely outweigh any disadvantage.