Sitting at the feet of the master may engender a warming glow, but have you ever hung around with someone whose expertise is the extent of his or her capacities, who basks in little else and who is plainly incapable of addressing other human concerns? In-sufferable!
On the one hand, it is such a delight to find someone who knows a lot about what I know a little about and yet would like to know more. Wowsers!
But when these accomplishments seem to convince and immure the one who accomplished them, when the discussion of spiritual adventure, for example, seems incapable of telling a good dirty joke or admitting a deep love of chocolate or expressing an unencumbered desire for a date ... how limited! how self-centered! how borrrrrring! And the same is true for dancers or academics or successful businessmen.
But it is instructive since it is so human. Expertise and accomplishment may be what anyone might seek, but when the one who actually achieved them is convinced by his or her achievement, when the best they can do is bask and rest and claim some elevated position ... well, isn't that the height of foolishness?
No one who has actually achieved something after long and arduous effort imagines s/he has accomplished anything. The new-normal of expertise is just the new-normal ... and the important part with expertise is that it is never the end of the trail. When it is the end of the trail, then something is missing.
From this I think it is fair to deduce that what anyone knows is not nearly as instructive as what they don't.
Here is a BBC posting that addresses the 10,000-hour rule of thumb for expertise.