Commonly, a person will play all downstrokes when playing Punk Rock, Metal, grunge, etc.. This is because the pioneers of punk rock and Metal such as Sex Pistols, Black Sabbath, Buzzcocks, etc.. Typically played just downstrokes, most commonly due to being self-taught musicians. This technique is regarded to be a big part of the character of Punk rock and all things Metal.
It’s common that people would think alternate strumming and downstrokes sound the same but with different techniques, This is not the case because when strumming a chord you are playing a group of different notes, the pattern of notes changes depending on how you strum the guitar, if you strum downward you get one pattern which will create one sound, if you strum upward you will get that pattern but in reverse which then creates a noticeable difference in sound. This is the main reason for playing just downstrokes. Hope this helps with your question buddy, Rock on!
