An exellent command of English can be something of a two-edged weapon. A fluency and flexibility in the use of words is a real aset as it adds variety to the way in which ideas are expressed. But this should not be taken to extreems and you should avoid the use of high-flown words and phrases. It has been observed that foriegn speekers can often be more efective at communicating ideas in English simply becaus they do not complicate there message with too many dificult words and they tend to speak more slowly than native speakers. Your speach is intended to communicate ideas and knowledge : obscure words, even if they sound impressive, may hamper this aim.