User blog comment:Rozh/Surface-to-Orbit Artillery/@comment-923138-20110320203751

I'm reminded of this quote from the commentary:

Marcus: (On the Onager) Now, my question is, how come we haven't had this gun, like all the way through the four Halo games, and now finally this one shot...

Marty: 'Cause there's only one of them in existence, and it got blown up on Reach! (All laugh.)

Personally, I'd say that the concept sound interesting, but likely would just be implemented on the small scale. Something like the Onager can be made relatively easily, and serves a useful purpose in defense of ships taking off, and thus can be cheaper seeing as it's expected to be destroyed. A large scale ground MAC has a more permanent use, and thus would ensure the majority of the enemy forces will be concentrated on destroying it. A ground MAC would far much better a surprise weapon, and for that something small like the Onager would probably have a more efficient use.