You wrote:
So people can write special software that gets their message rejected by an entropy filter. This is a disadvantage? It looks like an irrelevancy to me.
It's an artificial example, but one that points out that merely doing a frequency analysis on the datastream isn't enough to guantee the correct answer. Reliable remailer software will have to worry about false postives as well as false negatives; especially if it's a fee-for-service operation. This might also be a nice feature if you're trying to dodge an NSA filter.
Seems to me that a quite reasonable condition of use of a remailer is that what is passed isn't human readable.
Of course the implicit assumption in that statement is that encrypted traffic hasn't been outlawed or regulated, or that the sender doesn't want to 'appear' to be sending encrypted traffic.