Maybe this will help visualize it...
Here's a link to a diagram of the skylobby/local elevator concept in the WTC:
http://en.wikipedia.org/wiki/Image:W...Arrangment.svg
The graphic on the side shows you that by using the skylobby system you can provide the same level of elevator service, but that many of the individual floors don't need the full amount of elevator banks.
Here's some stuff I just scraped together for the Sears Tower.
This is the 5th floor.
There is a bank of 8 elevators that make up the 33/34 skylobby.
There is a bank of 6 elevators that make up the 66/67 skylobby.
There are 4 (numbered) banks of 6 elevators each
The red dots are the two express elevators that go from the basement to the skydeck and make no stops in between.
The blue dots are the service elevators.
On the 5th floor you can see that only elevator bank 1 is used. The "lobby" for the 33/34 skylobby is used as storage space (or maybe a reception/entrance area?), as are the "lobbies" of banks 3 and 4. Not sure what's going on with the SL66/67 - it looks inaccessible.
Now on to the 19th floor
The SL33/34 "lobby" is still storage (conference room?). Bank 1 has completely disappeared, replaced with bathrooms. Bank 3 is gone too, with 2 storage spaces and a walkway. Bank 4 is used to get here, and the SL66/67 "lobby" is now part of the rentable space.
41st floor
SL33/34 is gone (stopped at the 34th floor - with the 35th as the mechanical penthouse?). Bank 1 is back and in use. The lobbies of banks 3 and 4 combine for storage or something, and SL66/67 is a room of some sorts.
43rd floor
Two floors up and things change! Bank 1 is gone and replaced by a large room. Bank 3 is now used and the "lobby" for bank 2 is now storage. SL66/67's "lobby" has a new configuration too.
82nd floor
A big jump up towards the top and the building has now gone through the first two setbacks and is shaped like a cross. It's a little interesting now - of course the 66/67 skylobby is gone. But they chopped the top 2/3 of banks 1 and 3 and the bottom 1/3 of 2 and 4. This creates two new elevator banks, but only 1 is in use. The "lobby" of the second is used for bathrooms. Oh, and it looks like one of the passenger elevators has switched to a freight elevator, which is probably good since we only have 1 of the original freight elevators left.
95th floor
The remaining setbacks have occurred and the building is in its final shape. There's only 1 bank of passenger elevators left, the other is now rentable space. If it didn't go away, there'd be no east facing offices on the 95th floor. And that would be a tragedy!
The truth is that yes, 1 large elevator shaft takes up the same cubic space as several shorter ones. But you have to remember that as the Sears Tower rises, the square footage of each floor shrinks, and space becomes more important. Down on the lower floors where you have the gigantic floor plates, the space closest to the center is less desirable. Why not use it for the skylobby banks and free up extremely valuable space on the higher floors? Since you're doing that, you might as well stagger the "normal" banks and free up space for some bathrooms or storage. An added bonus is that your personal elevator experience (once you use the skylobby elevator) is that of a low-rise building since no elevator serves more than 10 floors.