They do not have a preferred element. It's not optional. It's required or they won't get the skill effect. The only reason to use off element weapons is if you're new and need something to fill in temporarily. Using an off element weapon because of its type does not make up for the loss of its skill.

Unless you're a hardcore whale with several completed SSR weapons to choose from, weapon type is not something you will have to worry about for several years of playing. Even free players who've played from launch still don't have full SSR weapon grids unless they got lucky with gacha and even then, many event SSR weapons don't have assault.