[2014 update] There are two advantages in having TTL-capable speedlights for off-camera use, if you also use TTL-communicating radio triggers: you can have true high-speed sync off-camera, and remote power control over your lights.
TTL capability doesn't just mean you have TTL as an auto power-setting mode. It means you have the full hotshoe/flash communication protocol. With triggers such as the PocketWizard Mini & Flex units, RadioPopper PXs, or cheaper TTL units from Cactus, Phottix, Pixel, or Yongnuo, you have the convenience of being able to adjust the power from the camera. You may also be able to set groups differently or turn groups on and off, or set zoom, sync, and even custom functions from the camera menu as well. But for this full function, you must have TTL-capable flashes.
The push is now on by 3rd party manufacturers to add remote power and group control to manual lights as well, but it's rare and typically ties you into a specific flash & trigger combination (e.g., Godox V850 or Wistro barebulb flashes and the FT-16/FT-16s combo; or YN560iii and YN-560-TX). And HSS cannot become part of that equation, given that manual flashes by their very nature, are not capable of it.