I was shunned at a group ride because I was not on a road bike. At the time all I had was a hardtail MTB that had slick tires on it. It seems as though most roadies think that if you're not on a road bike you shouldn't ride with them at all.
I've seen this happen with my club, too. Usually, early in the season, some new members, unaware of what a "moderate" road ride actually is, will show up with the most inappropriate equipment for our weeknight 40 mile rides. Usually, we just give them an easier route to follow, then we go on our own way. Once in a while, though, somebody tries to ride with us.
After 3 or 4 miles, though, the group either has to drop the guy and forget him, or else we have to slow our pace down to keep from leaving him behind. Once, this meant that we were only able to do a 25 mile loop. Now, we usually just tell MTB'ers that it's their responsibility to keep up with us, and then show them the re-group points on the map. None have been able to keep up with us since then. Just having tall enough gearing and road slicks does not make a MTB as fast as a road bike.
I don't show up at MTB rides with my road bike and expect the dirt riders to slow down to my speed. Why should they expect us to slow down to their speed?