to hide a signal you need a lot of noise.
if every beep is a signal there is no privacy.

so to build a working privacy-setup,
you need to make it obligatory.

only if noise and signal comes in the same stream
there will be obfuscation.
 
// if every transaction via darkSend is a transaction that is worth investigating - the fact that it is send via darksend becomes an information.
// if you cant tell why it was via darksend (`?was it becouse it was obligatory or did the sender/reciver needed the privacy?) the darksend transaction is not an information.

// by making it obligatory you a) create more "false positives" and b) create more darksend transactions overall -
// making the investigation a lot more hard work.