FWIW I don't feel that one should ever really do away with PWCs completely (unless you don't plan on ever adding anything new to the system and even then the logic is questionable). At one of our fests, Bob Fenner spoke about many things but the talk mainly centered around reef chemistry and things you should do to acclimate new additions into your system. One of the factors was that chemical warfare within an established system sort of balances out and bringing in something new 1) the new addition needs time to get used to that chemical mix and 2) the existing inhabitants can (usually will) react to the new addition. New chemical boundaries are set up and this is a stressful period. Limiting that stressful period by non-contact acclimation (meaning, not in the same physical space but in the same water system) allows for both sides to adjust.
Now, not doing water changes for a prolonged period of time means that whatever is in the system is used to the chemical soup that it is in, but these chemical boundaries are harder to "break" through. Doing water changes breaks this down so that new additions can more easily adapt and the existing inhabitants can more easily accept new additions.
That's basically the gist of what I learned. I probably missed something or got it wrong but I'm pretty sure I'm almost there.
Not to mention, if you dose, you're eventually going to skew some parameter and there's usually 1) no easy way to tell exactly what this is without lab grade test equipment and multiple tests over a period of time, and 2) probably no easy way to "fix" that, at least one that is significantly cheaper than doing even a minimal 10% PWC per month.
But that's JMO :)
