Go ahead and imagine Garchomp with Mold Breaker. I'll wait.
1. Ground types of course have a HUGE advantage from Mold Breaker, as probably have of the pokemon that Mold Breaker is useful against have Levitate.
2. Pure Dragon does not have that much from Mold Breaker as there is no ability having to to anything with Dragon (not even Solid Rock as of now).
3. Even if I imagine that, you're argument is pointless: It would come down to Garchomp killing Bronzong (again). Everything else are risky switch-ins at best. Let's go thorugh the OU list...
(a) Azelf, dies to EQ now. Would have died almost to Dragon Claw (good chance after residual damage), easy thing for outrage.
(b) Flygon, dies to EQ now (?). Would have died to Dragon Claw, too.
(c) Gengar, dies to EQ now. Would have died to Dragon Claw, too.
(d) Rotom, dies to EQ now. Would have been able to revenge kill (NOT switch in) at near full health (perhaps).
That's it from current OU. Surely, Mold Breaker is nice to have / a good ability / better than Rivalry if you're not into playing strange mind games / eases prediction in many cases. But come on. It's not Magic Guard, it's not Intimidate, in the current metagame, even Levitate is more useful. (Though THAT might change easily.)
e: Also, seriously, Mold Breaker not as good as intimidate? Yeah, let's see Salamence take down that Bronzong with a fierce look.
Come on, don't make me laugh. I just stated that Bronzong is the ONE pokemon a Dragon really needs Mold Breaker for. On the contrary, I'd like to see Mold Breaker Gyarados switch into anything... >_>