It depends on whether you or the other person has strong feelings for the other person. If you do then the harm is that you are treating the other person like they are cheap. You're or they are limiting what they will do or offer the other person. In my book that is utterly insulting. They are good enough to sleep with but nothing else.
Please, don't put yourself in a position you are not comfortable with :) If you already have done it find the way out saving everybody's feelings. Secret dating has no bright perspectives, unfortunately. If anyof you is in another relationship and you still have strong feelings, clear your relationship status first, then make the secret dating official :)
The biggest harm is that the secret will not be a secret anymore! Depending on your situation, the revealing of the secret might harm your reputation and maybe even other's view of you. People might also feel hurt since you do not want to share what's going on in your life with them, especially if they are family or close friends. You could also most likely hurt the one you are dating too especially if he or she does not what the relationship to be secret anymore!
Secret dating.... This means that you are dating someone in secret right? Is it because you are technically not allowed to have the relationship or are you ashamed of the person that you are dating? If it's the former then you had better stop right there because the truth will be found out in the end for the reasons specific to you. But if it's the latter reason, ask yourself why you are ashamed of him/her and if you want to take the relationshp further along. Don't lead someone on and then hurt them.
Dating someone in secret puts a lot of strain on the relationship and can limit the future possibilities of where the romance may lead. It calls to mind the question about why the affair must be kept a secret? Who would be hurt if your relationship was discovered, and how is the secret protecting you or your dating partner? In most cases, it would be better to address the root cause that’s leading you to secretly date someone (troubled marriage, disapproving family, etc.) than to continue in secret. After all, what if you really like this person and then have to reveal the secret later on?