subkeys on smartcard?

[Daniel Pocock]

I understand this is a bit old, but I believe the concept is still current:

http://www.gnupg.org/howtos/card-howto/en/smartcard-howto-single.html#id2507429

Essentially, can anyone confirm why it is recommended to only store
subkeys on a smart card?

a) is it because of the risk that the card might be lost or damaged?

b) or is it because of the risk that a smartcard may fall into the wrong
hands?

If the primary key was generated on a PC and stored onto two smart cards
and they were both kept in secure locations and only used in a secure
location, is it therefore quite acceptable to use the primary key on a
smart card and potentially more secure than keeping it on a disk?