Knowledge was power, and Piotr would never say no to it. He wouldn't push for the recipe and directions, but if Cas happened to think of it again later, or the matter came up again, he wouldn't balk at it, either. Even if they never saw any other angels for the rest of their lives, it wouldn't hurt to know.
Just like normal people probably didn't think that having a good recipe for napalm was practical.
"I do not wish to test what I can do against what an angel can do." Piotr admitted. "I have had much training, I can do a lot with my abilities. But I have not had to fight something actually divine. Pretenders, supernatural creatures... real angels, though? Does not seem like very many would survive that kind of encounter."
He did wonder if there were things that Cas could do, even know, that he wouldn't stand a chance against. Piotr hoped he never had to find out. Or at least, that he wasn't ever on the bad end of it.