Consider the boom in heraldry-themed trinkets over the past few decades... Yes a few people DO have coats-of-arms these days, but even those are merely curiosities (I highly doubt ANYONE today has a coat of arms because as a noble, they may be required to raise and lead an army on the field and need a personal device to paint on their shield to identify them during battle!). But plenty of people proudly display coats of arms borrowed from some historical figure that just happens to share the displayer's last name, whether they realize the bogus nature of their use of the device or not. When I hear some keyboard scholar telling some 'plastic paddy' that he really shouldn't have the Kelly coat of arms on his key chain because the coat of arms really isn't his, I think of an in-law of mine whose last name is Case...he wears a belt buckle from the Case Knife company because what-the-hell, it has his name on it!
In the long run, what harm?
Respectfully: the "harm" is that arms belong to individuals, not surnames, in the heraldic traditions of the British Isles. If your point is to honour your ancestors, then it is bad form to usurp arms that do not belong to you. This "keyboard scholar" really doesn't care what someone displays on their key chain, but as an educator, I'm not going to hide the truth from them if they ask -- if they don't, then fine. The intentions may be honourable, and by all means, display the arms -- just don't claim them as your own.

Its nice that we have so many scholars who have sorted out the facts from the fiction for those of us who really care, but if your goal is to enlighten those who have absolutely no desire for enlightenment, I'm afraid it will be nothing but tears and headaches for you in the end.
Welcome to the world of a teacher. Of course, most teachers worth their salt will tell you that you do not teach for those who do not have the desire for enlightenment, but rather for the ones who do. Apathy and lack of desire should never be a deterrent against education. Better to seek out the one who does care than the 99 who don't...

T.