I agree. You do use the theory far more than one might think.
Was using some Gen Nav stuff the other day when we discovered the course on the plate and the FMC disagreed. A solid theoretical underpinning is important, though some areas are more so than others.
Thread drift I know, but the areas I think are most important are, in no particular order
Principles of Flight
Law/Ops limitations
Performance
Met
For example, I got the Swatton aircraft performance book after reading a discussion here recently and was seriously enlightened. Has led to some serious learning and interesting conversations in the cruise.