I'm going to take a crack at this.
First you need to understand what pH is. The pH of water is the ratio of negatively charged and positively charged hydrogen ions that water contains. This said pH is not really a water parameter but a product of the acids and bases that the water contains. Think of it as a tug of war between acids and bases in the water, with the "knot" (which is what you measure) being your final result. If the "teams" are perfectly matched, then the pH will be at 7, if the acids are just a tiny bit stronger then the pH would be 6.8.
Now many are under the assumption, that I can just change the balance and get a "better" pH for my fish. This for the most part doesn't work. Stable water's pH ALWAYS wants to return to its balance point, (whatever that is). Therefore when the acid from a pH reducing agent is added, the initial result is a lower pH but over the short term, the bases in the water neutralize this new acid until the stable point is reached. Please note the acid component is neutralized but the dissolved solids remain meaning that your water is now "harder" then when you started. Not what you are trying to accomplish for soft water (low pH fish). Moving the pH up is more straightforward in the aquarium. Sources of acids should be limited (and weak) in most tap water. These acids can be overcome with the addition of carbonates and bicarbonates. The kicker is that it is still an ongoing process where bases that are neutralized and change water should be replaced (or adjusted). The increases in hardness are not as large a concern as high pH fish enjoy hard water.
When do I need to worry about pH?
When it changes is one short answer and more importantly, why it changed. With tanks suffering from OTS, pH falls (and fluctuates) due to organic acids neutralizing the bases in the water. It takes a lot of acid to neutralize a base, this means the tank is producing a lot of acid from a lot of decaying organic matter. A lot of decaying organic matter means not enough cleaning and water changes. Not enough cleaning and water changes means that disease pathogens, dissolved organic compounds are at high levels, in short, a polluted tank. Its not the fluctuating pH that does in the fish, its the pollution that is causing a fluctuating pH that kills the fish. Even in a relatively young tank, pH fluctuations can mean that waste production (and acid, from the breakdown of theses wastes) is at critical levels due to overstocking. When faced with a falling pH, the first most important thing to know is what the kH is and is it falling. If kH is falling, increased water changes and increased cleaning with reduction of bioload is of utmost importance.
The second reason to be concerned about pH changes is changing levels of hardness. Through various biological means fish manage the levels of salts in there bodies, to keep a constant ratio. When the level of solids in the water changes dramatically, fish can go in to osmotic shock. This is where the admonition of never changing the pH more then .2 per day comes from. Again its not the pH change but the change in water hardness that affects the fish.
What about CO2? The addition of CO2 to water produces minute amounts of Carbonic acid (a very weak acid). This temporarily changes the pH of the water when present. (Please note, in the first instances we added a solid to the water in this case only a gas, the hardness of the water does not change). In stable waters with low amounts of acidic compounds, a pH drop of 1 means that the water contains approximately 30 ppm of CO2. Fish depend on the ratio of O2 and CO2 in water to facilitate the dispersion of CO2 from there blood through the gills into the water. If the O2/CO2 ratio is too high in the water, CO2 builds up in their blood and they suffocate.
Therefore in tanks with supplemented CO2, the hobbyist is concerned with the pH level since it is an indicator of CO2 levels in the water. As CO2 enters or leaves the water, water hardness does not change (no osmotic shock). This is why in a planted tank, you can do a 90% water change and the pH could rise by a full unit and the fish do not notice. Change the pH by a full unit by chemical means and your fish are floating in the current due to osmotic shock.
So can I just change my pH to benefit my fish by adding CO2? I don't believe so. The addition of CO2 changes the pH of the water but does not change hardness. Low pH fish like soft water, and adding CO2 to hard water does not change the hardness. In most cases many fish eggs do not become viable due to high levels of salts in water, not due to pH. You also have to be aware of the O2/CO2 ratio as mentioned earlier. In a planted tank, the addition of CO2 should increase the levels of O2 due to plant photosynthesis, keeping the ratio within tolerances. In a nonplanted tank, CO2 addition will decrease the ratio increasing the chances of suffocation.
So why does everyone go on about pH? The first reason is it is easy. A pH test kit are easy to use cheap and readily available and people think they know what they mean. This means there is a lot of profit in it. The store sells you a pH kit, your pH is wrong. The store sells you adjusting chemicals. You use them and your pH returns to its original point. You buy more chemicals. Your tank is turned into a chemical soup. Your fish dies. You buy more fish, you buy more chemicals, you buy more test kits. You get the idea. Second reason, my pH changed and my fish died. This is true, but in every instance I've ever seen, its not the pH that killed the fish, its the reason the pH changed. See the difference between causation and correlation. As my stats professor explained (to make a point), in the summer people eat more ice cream, in the summer drowning deaths increase, therefore ice cream causes drowning.
This is the abreviated answer
. There are good reasons to test pH, you just need to understand, what the pH reading means and act accordingly. The pH change due to gas content is meaningless to fish (unless you suffocate them), if your pH is changing for another reason, then you should probably figure out why. If you want to tailor water to your fish, don't use chemicals, buy an RO unit and build the water. You can always increase hardness with relatively less work, you cannot reduce hardness by adding stuff.
HTH