Granted, it usually doesn't seem to work very well, which supports your uncertainty. My experience is that general advisors mostly help bureaucratically, by making sure undecided students progress towards graduation while they explore topics. Which is obviously a bit short on real-life analogues, since beyond food and shelter there aren't many predetermined 'requirements' shared across all goals. But it at least has me wondering about the possibility of seeking a different style of mentor. Perhaps someone who's moved through several fields, or someone in a potentially-interesting domain chosen less for excellence than for their ability to give you an efficient taste of the field? If you're as far as a tentative goal this becomes fairly easy, as with undergrads doing research work or prospective law students doing paralegal/law-office work.
I don't have any great answer here, though, and I'd really like one. There are several things I'm interested in which seem to either lack this sort of pre-entry mentorship or lack any experts at all. If my desired skill is "the intersection of X and Y", but I can only find experts in X or Y... it looks like I'm rather on my own.