You guys from the dark ages make me laugh!
My CO2 controller keeps the CO2 level the same no matter what the conditions. If I have one plant or 100; in high light or low; with one fish or 100 fish; high aeration or no aeration; day or night. Reliable, reliable, reliable, no questions asked just service!
I like to think of your input vs outflow system as a wood burning stove and a controller as a thermostatically controlled central heating system. Yes you can heat a room with a wood burning stove but it is hard to get things in balance what with opening windows when it is too hot and adjusting the draft at night and having hot and cold spots. My thermostat like my CO2 controller does all that for me.
Not quite sure how to respond to this - there are many people that use CO2 without a pH controller, and have very successful aquariums.
You are right in saying that a pH controller can keep the pH very stable, but at the same time, pH swings induced by CO2 injection are not harmful to livestock, in general. Rather, it is excessive CO2 that will (and too often) kill off livestock; a pH controller will help prevent this.
Keep in mind that it is not the absolute pH that is important either; rather, a drop in pH can be associated with a certain concentration of CO2 dissolve in the water column, and this is what is more important to note. This is mainly because pH can be affected by other factors, such as tannic acids.
All in all, if you have excess money, a pH controller might be a night item to consider, but I would not say it is absolutely necessary for a functional aquarium. In my opinion, it would be better to invest in something like a dual stage regulator, so that a pH controller would not even be necessary in the first place.
One other thing I forgot to add on my previous post (You should have an edit post feature on this board).
I can easily adjust my CO2 levels in increments of 2 ppm.
There is an edit feature; I have merged your posts.