You can't get an exact measure of CO2 by any method accessible to us. But the drop checker, used with known KH distilled or DI water in it, gives the best accuracy of any method that is accessible to us. If you are experienced enough you can probably judge how much CO2 you have by just watching the plants, but I'm nowhere near that experienced, so I need a method to actually measure what is there. And, the drop checker is that method.

What Tom was saying is that you most likely don't have enough CO2 in the water, or your tank water circulation isn't good, so some plants never see much of the CO2. The drop checker at least gives you a rough idea if you have enough.