I think it was, like was said, an attempt to get a dual coil humbucking pickup to fit into a single coil guitar without modifying the guitar's appearance too much. A good thing if you have a collectable guitar but play around a lot of noisy stage lights.
And the rail or bar I think was a good way to get a smoother, more continuous magnetic field as the strings are bent.
I think I also saw a normal size dual coil Bill Lawrence pickup that had rails instead of adjustable pole pieces at one point. Same idea, regular size.
Thing is, people nowadays don't seem to experiment with adjusting the adjustable pole pieces, lol. A pickup sounds too muddy or bassy, and they just change the pickup instead of lowering it and bringing the pole pieces up. And the opposite it true too - too trebly and they quickly ask which pup has more 'balls' and grind. There's a lot that can be done with adjusting heights.
When I was looking for a humbucker to fit my Strat, I saw some company's rail type pup and thought it would somehow have too wide a spread to maintain the 'focus' of the single coil. Not lengthwise, but in width. I saw the pole pieces of the single coil right in the middle of the pup and then thought the two rails would somehow be in 'the wrong place' under the string. A small consideration to be sure, but having come from a time when tweaking pole pieces was more or less the norm, I was aware how much difference height adjustment coule be; having two separate metal pole strips near the edges of the pup made me think the sound would not be under the correct harmonic.
Then I saw the Duncan Hot Stacks, and at least to me, that made the best sense - the pole-bars being right where the original pole pieces were. Good, loud (hence the 'hot' in Hot Stack) humbucking signal. That's what I was after at the time, being in various venues with questionable power and lighting.
I think Carvin did have a good idea with the eleven pieces too! But then again, most people don't spend the time to adjust them.