I really depends upon what you end up doing. I use what I learned in control theory A LOT
but that is because I am an experimental physicists and happen to work in a field where we often use PID controllers for temperature control, feedback loops for stabilizing oscillators etc.
It would obviously not be quite as useful I was working in e.g. string theory.
Although I must say that having some idea about how to determine e.g. if a system is stable can really come in handy even when working on more "abstract" theory.
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