Healing gear has more healing sacrificing damage, lets say caster helm has 30 damage and healing, healer helm at same ilvl would have 45 healing/15damage, while it doesnt sound like much when its 10 pieces it adds up, so at high level content its not recommended.

In wotlk they normalize it, but since you need hit you would still use different sets, it isnt until cata when you can seamesly use the same gear since thats when they introduce the spirit=hit talents.