I tend to use it more for being descriptive or funny, just because it sounds funnier to me, and I’ll always go for the laugh when I can: “Wearing red makes my dumpy self look like a fireplug.”
But when I’m talking about the object right in front of me, I’d say hydrant. Lifelong midwesterner, if that helps.
I don’t recall hearing ‘fireplug’ around Philly in the 70s. Doesn’t mean it wasn’t done, but ‘hydrant’ had to be quite common also. At that time in the area we also called them ‘Minute Men’ because they would be painted to resemble Revolutionary War soldiers. Something I first found out the hard way by parking next to one and not recognizing that it was a hydrant, or plug, or whatever.