If I remember correctly from previous postings..
Think of the voucher as a $6 coupon towards anything you want to eat. You can pay the difference if you buy something (or a couple of things) worth more than $6... but you lose the money if you buy under $6.
And I don't think you're limited to just snacks or carts, which opens up the possibility of buying kid meals as well.
Ideas:
Corndog from the cart on Main Street ($5.80)
Turkey leg from cart ($6)
two skewers from Bengal BBQ (pay the extra)
Chicken Strips at Stage Door (pay the extra)
two bakery items at Blue Ribbon (possibly pay extra)
ice cream treats at carts (buy 2)
churros (buy 2 or 1 and a drink)
You get the idea...
menus here
http://www.dlresortinfo.com/DisneylandMenus/dlmenus.html
http://allearsnet.com/dlr/tp/dl/dl_rest.htm
Now that I think about it.. might be nice to have just a few vouchers, not a whole dining plan though. I know they don't save you any money but it's one less thing to pay for when there.
Typically we only eat lunch in the park and a very occasional snack.