I think with the v2 upgrade on the 702, $500 is nowhere near worth it:
-160GB 2.5in HD ($175)
-usb2/firewire 2.5 in HD case ($35, I already have one, has a 30GB drive stolen from a busted laptop in it now)
-usb power pack (~$20)
4x the space for less than half price, and I have the option of bringing it or leaving it at home. you could set up an equivalent 40GB drive for less than $90. The extra bulk adds up to something about the size of a wallet, and is easier to keep cool.
to me that's more flexible, to some it's probably just extra crap they don't want to deal with. but I think it'd work pretty well in the situations where you need more space.
Also: flash is far less sensitive to heat than mechanical HDs...If the 722 gets as hot as some people say it does, I don't think I'd ever trust just recording to the HD. Have many people had HD problems?