"They are simply testing themselves and their abilities WITHOUT causing you any unnecessary embarrassment (or themselves for that matter if they got whooped). He wanted to put his money where his mouth was without making some spectacle event out of it (which always ends making people feel threatened, and people get nasty too quickly when threatened)."
that's the positive one, but some peole do this because they want to prove that their school is best among others, that he had beaten sensei 'A' of this art, arm-barred sensei 'B' of this art and so on. I'm not really against this because if you do with honest intention this will create stronger friendship among each martial art.
Now if it went any further than that (i.e. he suddenly proclaims that your dojo is wimpy cause he bested you guys…that's another issue
that's the problem with these so called 'dojo buster', they are nice at the beginning but turned to be an asshole in the end. If it happen then it's not humility anymore but more to cockiness i guess, this 'testing' attitude is a double edged sword to me.