So long as you're getting your job done, who cares about what you're wearing.
Con Potential legal issues
An issue with not having any dress code is if people take advantage of this by dressing provocatively it can lead to issues between employees (taking offense at what the other employee is wearing, or acting inappropriately towards them based on misread signals), which can result in legal cases. Whilst these issues are not necessarily the fault of clothing, it does add to the list of excuses people may cite. Better to avoid the risk by giving some guidance, which allows someone to step in and resolve the issue (with the backing of company policy) before things escalate.