One important application for us is using overlay icons, e.g. for version management. Think of tools like TortoiseSVN or Eclipse that display the version status right on top of the icon's item. For this you need to use either 16 color icons (overlayed with 8-256 color icons) or true-color icons.
256 color icons use a custom palette. If you were to overlay two 256 color icons, the results look awful unless you do palette transformations (and even with that, they look ugly). Being able to use 32 bit icons would make this much more easy. It is not feasible to pre-render all possible overlay combinations, because for some use cases, we even use two overlays (to indicate different statuses), making this a O(l*m*n) problem
resulting in thousands of possible combinations.