Would be great if Starbucks took up on the theme and made a card designed for sharing but allowed the creator to define a single purchase limit. I don't think a system like this could ever be completely locked down to abuse but at least it could take away the gift card thing.
Now that's a great idea. Have a card that is limited in certain ways: max purchase of, say, $5 (I don't know if $5 will buy anything in $tarbuck$, since I haven't been there in many years). Limit purchases to coffee (or tea) only. No transfer allowed from one GC to another. No more than, say, 30 uses per location.
It would be an interesting series of social experiments to limit the card in these various ways, and see which limitation helps the uptake, and which hurts it. Fascinating!
