learning to play guitar proficiently is an accumulative thing... meaning, when you move on to something else you should be able to apply what you have learned in previous lessons as well as integrate what you are currently learning. now... as for me, i have found after playing for 12 years that there are things that i've learned that i have never went back and used because it just doesn't fit my style... in turn, i've pretty much forgot how to tap into it. i have terrible modal recall, for instance. but i very rarely find myself writing a song or playing and thinking "what mode would fit best here?" because i have accumulated an understanding of the instrument that transcends the notes and theory that is written on paper... to me, that's where i wanted to be the whole time... i never wanted to be able to rip off every mode and scale in perfect steve vai'esque fashion like a robot...

but then again, there's the subjective order of the question at hand... it depends on what you want to achieve. steve vai wanted to know everything and be perfect at everything... and he pretty much is now because he never moved on from something... he just kept adding to it.