Is it necessary for someone to know every aspect related to sub disciplines? How often are engineers required to use them all?
Sub disciplines are useful, in general - they keep you from being a one-trick pony.
Other than that, no, it's not necessary to know every sub discipline - there are several that just don't want to stick in my head (such as wave propagation and Maxwell's gradients.)
But, there are several that simply satisfy personal curiosity, such as the crossover between Electrical Engineering Control Systems and Mechanical Engineering Control Systems (they are taught quite differently).
My advice is to learn whatever you can about whatever is interesting to you. If it is uninteresting but necessary, learn that too, but only to the extent that you can use it to get to the more interesting things. If it is uninteresting and doesn't lead you to something interesting, leave it in the dust. Sanity is worth more than money.