The penality is so minimal once you have a trained ranked in the relevant characteristics (INT, WP).
It is only natural to prolong your life a little bit when you face the hordes of chaos.
p78 - Roleplay Rulebook
"Whenever making a Channelling (WP) or Spellcraft (Int) check, an arcane caster must add one misfortune die to his dice pool for each point of soak value provided by any armour and shield he has equipped. Soak values from other sources, such as magic spells, do not impede spellcasting.