Nick wrote:
Thanks for all the great responses guys! That is exactly what I
had thought, that since the outer coax braid is grounded at the
antenna, and at the shack, and (in my case) at the center with a
lightening protector, then I just couldn't figure out why some antenna
companies, such as reliable DX engineering with their DXE-VFCC H05-A
Vertical Feedline Current Choke, strongly recommend such a line
isolator for verticals.

Nick, think about it. If you ground the feedline at exactly a
common-mode standing wave voltage node, the ground accomplishes
absolutely nothing. You certainly don't want to use "plumber's
delight" techniques in your ground system.
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73, Cecil http://www.w5dxp.com