Since critical damage applies to attacks and spells, I'm just not sure of the necessity.

If i add points on fate of wheel in critical damage in the middle of rogue abilities does count to critical spell too ?

Any critical damage on weapons, gems and the passive skills (wheel of fate) benefits all attacks and spells. The only critical damage that doesn't are the skill modifiers in active skills (since they only buff that one skill).

Critical chance works the way you were mentioning above though (separate chance for attacks and spells). The good news is that several sources of critical chance affect both (ferocity and alexandrite gems in accessories for instance). Most affixes on items are separate like rings can only have spell critical chance while amulets and belts can only have attack critical chance.

I hope that this helps!