There are a couple of reasons for a priming horn. I have some nerve damage in my hand and started using a priming horn so I didn't have to handle my main horn and its stopper twice. For me it's a struggle to pull stopper, pour horn, replace stopper. To do that twice isn't necessary when I can prime with a valve priming horn. My hand is somewhat improved, but now it just seems more efficient.
If I wanted to use 3fg in the pan, I think i'd use 3fg in my priming horn. However, I do use 4fg; one reason I do is that I can trickle 4fg into the vent of a dry-balled gun. I had the chance to help 2 fellows in the woods walk at Friendship with Null B in my priming horn.
This is not meant change someone's mind. Use what you like best. The priming horn is more efficient for me.
Regards,
Pletch