I think it might be an interesting idea. I but also think that what should be also implemented is that if two different jobs have the same (for example) job trait like PUP and Monk's Hand to hand trait.

If subbed and the jobs overlap each others trait they gain for their corresponding levels that they add together to increase the overall bonus they get.

Martial arts I PUP + Martial Arts I (from Subjob) = Marital arts II

Few lvls later Martial arts II pops up on main and subjob's Martial art II is attained = Martial arts 4

1 + 1 = 2
1 + 1 + 1 = 3
1 + 1 + 1 +1 = 4
etc

Makes subjobs even better.