A lot has to do with the variety of shooting one does. Increased variety of place, style, colour, subject and use means an increase in the number and value of keywords to find images efficiently.
Someone like me with a comparatively small library (~50,000) of primarily landscape and nature shots made at discrete and periodic times through any given year, is less in need of an extensive, hierarchical keyword list. I can find any of my photos from over 10 years of digital shooting on three continents, 15 countries and about 30 states/provinces/counties with two or three keywords and three or four clicks, max. And that's with a keyword library of just over 2,500. When keywording, I try to be as descriptive as possible with regards to subject, place, time, colours, conditions and details.
If I was to have a keyword library as extensive as Seth's, I would be spending all my LR time key wording! There comes a tipping point, which varies from person to person according to style, variety, number of jobs and how important it is to your business to find groups of photos on specific themes. You want the ability to efficiently find them across numerous "folders" without spending too much time entering keywords after each shoot. Thank goodness for autocomplete, otherwise the task would be more monotonous than it is.