IMHO, Joseph "Piney" Armone was 2 sides of a very interesting coin.
On the one hand,he was an old school, live and breathe,Cosa Nostra kind of guy,yet on the other hand,he went along with the hit on a sitting Boss.

From what I've read,he had ambitions beyond being a mere Capo.
He also didn't respect Castellano as a "real" gangster.

He was a great asset to Gotti in the sense that he could help smooth things over with the other old school,(and new school as well) Family members after the hit.
Piney was a well respected man,and lent some legitimacy to the new regime.
Armone wasn't going anywhere under Paul,and saw Gotti as a path to move up,as he eventually did.

Gotti needed Joe,and Joe needed Gotti.