I used to ride road bikes with drops, so I understand the multiple hand positions benefit and on my bike with bar ends, I often ride with my hands in a similar position to riding with drop bars with my hands on the brake hoods. However, I thought MTB frames generally had a longer top tube designed to be used with straight bars, so aren't the front of the drops or the brake hoods too far forward, if you see what I mean?
My preception is (only my perception and only as good as the research I did when looking into drop bar MTB's. You may find other reasons).
The original idea behind drop bar MTB's:
Design a cockpit where the rider mainly uses the drop part of the handlebars (unlike a road bike setup where the hoods and the flats on the top of the bar get used more often).
the benefit being a more ergonomic placement of the wrists, as opposed to the horizontal placement of flat bars.
A more ergonomicaly correct position of the arms/joints to work as shock absorbers adjusting for the bumps from the trail (visualize you physique riding in the drops as opposed to a flat bar).
The use of the spiral shape and the longer length of the drop bar handlebars compared to flat bars to absorb the vibration of the trail.
It looks unique and cool, and Jackie Phelan always had a dirt drop setup.
All Hype... possibly (I don't think so).
I think a lot of MTB drop bar bikes are set up with the intent of using multiple hand positions like one would on a road bike. As opposed to setting things up to use the drops as the primary hand position. Thus missing the benefits of the founders initial plan.
the design of the vintage WTB bars (and the others) from the 80's makes it virtually impossible to ride on the hoods and leaves very little area to hold on to the flats up above. The design encourages you to be on the drops below.
Just some quickly put together thoughts and things I have read on the topic to stir the pot and continue the discussion on a topic I enjoy...