I'd love to have this feature as well. But adding to that, there are a number of situations where you use one quest item to get another quest item, and then use the next quest item to get the final item you need. For example, sorry I don't know the name of the quest, but the cooking (or is it fishing?) daily in Orgrimmar where you use a knife to cut off the tail of a thunder lizard, then use the bit of tail to shock fish so you can collect them. The carbonite quest tracker has the handy feature of providing a button you can click to use the quest items. So, in the above example, getting the bit of tail is easy. But to actually collect the fish you then need to open your bags, find the tail bit, and use it from your bags. It'd be great if the item icon can be updated for the next item, or added below the first one or something to that affect.
Definitely not a big deal for me, but never hurts to make the suggestion!