The issues where P types slack is discipline. Finishing things is a way to externally measure competence as well as taking a test. The other way to show competence is to prove yourself when you need to. Most Ps I know are extremely competent, we are disciplined, but honestly I could careless what my worth is in the areas that Js care about. Honestly I know my worth in life everyone else knows my worth, I am 100% confident in it, and I dont need to measure it. Its what allows me to be P. I have never doubted my worth as a person.
You say "acting" like a P is irresponsible and ignorant. At times I will be extra P around J types just because of the impression it gives them that I am incompetent. But its funny that I actually impress people because of my competence and knowledge despite the fact that I am a P. Its one of the ways I play with people, its one of my P traits