WIY- i am not going to debate what selichos is all about but I am going to tell you that “achdus” is indeed a most important thing in Klal Yisroel. You will find many instances where achdus was the reason for sucees . See matan torah (Vajichan ho-om tachas ho-hor) and, lo aleinu, the opposite too (dor haflogoh). The point is that to bring Jews together in a mitzvah (you are not seriously going to say that the selichos were not a mitzvah?) is in itself a major reason to do it. So, regardless of your thoughts on selichos, the achdus aspect was very important.
