Well one advantage of using TTL would be squeezing every last drop of hypersync/high speed sync out of tiny speedlites to balance your flashes to sunlight (ie sync at ridiculous >1/1000th second shutter speeds). This has been something of a revelation with the TTL Pocket Wizards, enabling wide portraiture apertures whilst using the sun as a backlight and so on (like you can do with a 'portable' studio flash and powerpack).