Paradox Update ----

It will allow the command block to receive more than just commands, but input --> Output commands. For instance, the command block asks the player(s) to input their usernames as they are on Minecraft.net's server. Once it does, it builds an array of the player(s) and then that command block uses a return function into the next block that processes that information. You'd need a special dust to carry the output from the input box, probably aqua-colored redstone dust? (Diamond Dust?)

Once the next block receives the data, you can link it to a modification such as Citizens and output a citizens NPC based on the player(s) that typed their username(s). This would allow for scarier things like Doppleganger bosses in adventure maps, or if you go farther into a map, you can see yourself doing the earlier puzzles (IE: Time warping adventure maps)

it's very complex I know, and very creepy; especially if you do a ghosting system via pressure plates (or ghosting command blocks that would throw what coordinates were walked upon to a command block that would go through the aforementioned generation of a doppleganger and then have it path along those coordinates, etc.)