I don't wear one routinely, because I usually shoot recurves that have relatively high brace heights. If I’m shooting a longbow with a brace height less than 7 1/2”, I usually get string slap and have to wear one.
Some people, women mostly, have an elbow joint that bends slightly backwards at full extension, and they generally need to wear an arm guard unless they can learn to keep a slight positive bend in their elbow joints.
Shooters of all skill levels either do or don't use arm guards, and I’ve never tried to encourage any of the people I teach to use one or not. I just always have one available in case they want to use it.