I have just worn socks for some stream crossings, or an old pair (1970's vintage) Nike lightweight running shoes that weigh a pound and double nicely as camp shoes. The new Free 5.0's sound like a reincarnation of that lightweight design – about time too since my old pair is falling apart!
Update: In the quest for a better stream crossing solution, I just came across a lightweight 2mm neoprene sock with rubberized grip pattern on the sole that should work perfectly for wading. They are called "Sandal Sock" (the brand name is Warmers) and REI has them in the kayaking/canoeing area for only $15. Weight for a pair (large size) is 3.9 oz.
I think they will provide reasonable foot protection and a bit of traction for any streams I need to wade across, and may even do double duty as a camp slipper. It doesn't look like they will absorb much water, so unlike my socks or old Nike's they should weigh about the same after wading as they do before wading.