The Traits can be split into two groups currently: "Ability Enhancer" and "Stat Bonus"

Ability Enhancers are what make a skills better for their originating class. If these are allowed to passively(i.e. automatically crossclass and not taking up a slot) cross-class then they could inadvertently cause one class to be more effective in its roll than others if allowed to grab the right cross-class skills. One the other hand most are weaker than another full skill if you have to put them in an add slot.

Stat Bonuses are to risky. One or more +2/6/12 stat bonus have the potential to be the most optimum add skill if you need to select them and passively granting the bonus will potentially cause players with every other class at 24 or 32 to have far greater stats than those with only a single class at 50.