It's what makes him worthy of being king. The others are just lowly power mongering fools.
Not really. Kaidou and BM do it like that, well probably BB as well, but we know of WB that he simply let them use his flag because Neptune is his friend, that´s all, and Luffy is the same really.
There is a small diference though. WHitebeard kicked everyone's ass, expelled every pirate that was hurting FI, and protected FI. Luffy did half of that, but BM's flag is still up there, and the day it comes off, Big Mom better not be alive to hear about it.
The point still stands though, some Yonkou reign like actual emperors, some simply let others use their name for protection. Luffy will never reign over territories, even if BM gives FI over or something, heck like he said to Rayleigh, he does not plan to conquer anything and even recently, that point was made clear again when he initially refused to have "sons" under him.
that is well and good but........well he does....... he already planned on giving FI his flag and they would come to his aid if he needed them. he does have a fleet.
so regardless of what he says he is well on his way to owning a fleet and many islands