Boots are made for safety, not looks. They keep the door from sliding through the stirrup, espicallg since most beginners tend to keep the stirrup farther back on their foot for comfort
Boots are there for a reason. They're designed to help prevent the foot from sliding through the stirrup. If anything, beginners should definitely wear boots as they're not going to have the control over their lower leg that a more experienced rider has, and may put their foot through the stirrup whilst walking around.