Nobody will stop you from using what you choose. It's just that nobody with a lick of intelligence will buy into what you are saying if your biases are both so rigid and transparent. If you acknowledge that ERA is a problematic because it doesn't adjust for anything, and then criticize another stat that does adjust for things, but maybe not all of them, and then say that you're just sticking with ERA because the other ones aren't perfect, it's like saying that the Model T was the best car to choose from because it gets you where you are going and the other cars don't yet partake of flying or space travel. Yes, the ultimate car might be able to fly or go to space, but it doesn't mean that improvements like air conditioning, power windows, better brakes, etc., aren't ACTUAL IMPROVEMENTS.
A 9 year old could say "But, dad, doesn't it matter that Hammels plays in a different league?" or "It seems strange to me that the best way to measure one player vs another would be based on a counting stat like earned runs, when the ballparks and competition one pitcher faces is so different from another. There must be a better way."