Have you ever breathed through your nose while speaking.Try it now ,the result is you are not able to do that.You cannot talk while breathing through your nose.The only way to breathe while talking is through your mouth.
Breathing through your mouth is considered a bad habit because mouth is
not designed for that and should be used only in the case of
emergencies.Talking is not considered as an emergency but is the only
way to breathe while talking unless you take small intervals while you
talk and breath through your nose .The choice is yours.