Bottom line is if you cycle without a helmet you have not properly assessed the risks.
Don’t really want to get into a pro/anti helmet rant. But one person’s assessment of the risks may differ from anothers – since it kind of depends what personal emphasis you put on the various risks and outcomes.
I do wear a helmet when riding, but if someone has assessed the risks and decided not to then fair enough (but if they just don’t want to wear one because it messes up their hair or because workmates might laugh then they are an idiot).
Bottom line: asses the risks for yourself and decide if you should wear a helmet (or any other protective gear) – but don’t give a crap what your workmates say, it’s your safety not theirs.