I was thinking earlier about the legal system vs crypto. Does anyone think user defined mixing will be a weakpoint of Monero in the legal department? As in, a fixed mixed count would have been preferable if attempting to design around the law when user defined is pretty similar to active mixing.
It seems kind of obvious that governments would be far more likely to consider active mixing as an act of laundering (darkcoin), while if anonymity is part of the protocol itself, it's just a shortcoming of government auditing. Both Monero and Zcash are safer legal-wise (than Darkcoin), although you would likely have to enforce a fixed mix count instead of variable if you really wanted to be safe in Monero.
I don't know the legal side of things, but in terms of the current protocol, the default fixed mixin count is the current implementation.
I.e., if you type
transfer
gingeropolous@payxmr.com 500
it will send me 500 xmr with a mixin (or ring size) of 3.... and nowhere in the command is there a 3 specified.