well i agre it all depends on how much time you want to sopend on them, but to me spending so little time on each makes it not worth doing them, unkless they are for plkay, like art appreciation.
even in undergrad school, some of us began with grad courses, i enrolled in ym first one as a firsts emester freshman, and my more advanced colleagues took them for several years as undergrads.
so for advanced science students, it is often advised to take grad rstehr than udnergrad courses, and then they would be facing the same time demands as grad stduents, onlky without the commitment.
i.e. a successful undergrad career at the high level, often involves taking grad courses, and one needs to do well in them.
anyway i do not agree that rgad courses are so much harder for the grad student than undergrad courses are for the undergrad. i just do not believe there are many undergrads out there who can profitably take honors calculus ala spivak, and honors physics ala feynman, honors chemistry, plus honors writing, maybe a language like russian or german, and also a demanding philosophy course which reads a book or more a week like the illiad or republic.
these were the kinds of courses we took as freshmen at harvard, and i think we were taking too many of them at 4 1/2, much less 6. i gues it is my turn of mind but i like to think about what i am taking, not just listen to surveys, and never get into it.
i.e. to me learning is about understanding, not just exposure. and i think for most undergrads, the last two years are also about deep investigation, maybe writing a thesis. 6 courses at that time seems silly to me.
in fact i question how much oppenheimer got out of his vaunted 7 harvard courses. i tried unsuccessfully to see a list of what he took last night. but apprently he was a voracious reader who did little else but study.
in my day too, there were a few people who entered the library in the morning and left only at night when it closed, every day. this may even be thought unhealthy, and i worry abiut some of our young members here who speak of nothing but study.