I think more useful would be the ability to also send small, twitter-sized messages instead of just transactions via darksend. Imagine that you could combine a transaction with a message or something like that. Hash the characters together and send it like a transaction, and all of a sudden, you have a really well encrypted, anonymous message system that overlays on an existing network.
This compromises anonymity (unless the message is utterly generic, in which case there's no reason to send it). You could argue that it's one person voluntarily compromising his/her own anonymity but because of the way DarkSend works, they're chipping away at the anonymity of everyone else involved in the mixing.
I don't think you get it, the message would be encrypted with the coin or address or something, so that when the receiver gets the coin, they can see who it came from and why or whatever short message you want to send, a purchase order reminder, etc...
Certainly the person receiving the message should know why the money arrived, just not the rest of the world!