Well, like all topics of debate you have some who wish to explain & convert, or at least get someone to see your POV. Then there are those who would rather not influence others and feel no real duty to explain it, especially if they are hostile, or rude about it. Which I am...
I prefer, however, to tell if asked and only when asked. Of course I only mean this in context of someone who is curious about it and wants to know more.
However, If I am..say trying to date a mono and she needs to know that I am married and poly, then I may try to be more persuasive with my POV. Depends on the situation.