Although I personally don't like casters having access to Para/Silence, 5% is so low I honestly can't see it being any sort of issue. Not when there is things with poison resistance to counter them activating at all.
So 1/20 hits might Para or Silence pop a purification potion no issue that's if the 2-3 seconds your Para'ed or Silenced would have made a difference at all. Taking into account the chance of being effected it's very unlikely you would even need to use a 2nd purification potion during it's cooldown. Except if fighting a Assassin or Warrior who have natural abilities that can stun.
The only other exception for this would be large scale fights e.g Sabuk wall and if your being hit so many times you get multiple para'ed then you probably would be dead either way.
Just sounds to me like you making a mountain out of a molehill.
"who saw it for the wizard to have paralisys on skill ... 5% is too much for me"
Jamie did on Zircon and it was part of his files when he released them.