[vipBrooriErok]

there is no "need" for adding "khairan". There are many arabic statements which have mahzuf (hidden) words in them and they do not make much sense on their apparent. for example what does Ahlan Wa Sahlan mean?

More importantly those who feel that it has to have khair as opposed to shar, then there is an Usool which we follow. A Mu'min does not hope shar for a fellow mu'min. So we do not perceive a muslim opting for anything but khair for other muslim.. hence no need to add it. But if you add it, then no need to remove it