Marlin abandoned the Micro Groove about 10 - 12 years ago in favor of Ballard rifling (deeper - wider)for better accuracy with cast bullets, and I expect for copper jacket ones also.
The first couple of Marlins I owned in .45-70 had the Micro Groove, the last 2 had the Ballard and I would have to say that they were more accurate... like 1" - 1 1/4" with most loads. The barrels on the NEF and H&R also had ballard type rifling and are/were very accurate as well. I also found that they cleaned up very nicely from copper or lead with less bother.
I suspect you are right about costs, though I have no proof of that. It may have been just a selling point for the unwary!
Bob
www.bigbores.ca 
