Darth Vegas
Sin City Sith Lord
First and foremost, the main reason why writers don't have Tony make an armor out of pure adamantium is because it would basically make Iron Man almost completely impervious to harm (aside from King Thor's eye beams and Magneto's powers).
That would make the character extremely boring b/c he would basically be unbeatable (a la Pre-Crisis Superman). There would be no drama whatsoever.
All of those "Buster" armors are nice and all but to me, they're simply plot devices.
The best moments that I enjoy about Iron Man are when his armor malfunctions or when it becomes severely damaged because that's when you see the truly best part of the character of Tony Stark emerge; seeing him use his intellect and ingenuity on the fly. That part of his character is what I enjoy the most; seeing how he comes up with ways to defeat opponents when he can't completely rely on his armor or when he knows that he's clearly outmatched (ie. using CLOC to beat Sentry). To me, that's what so great about the character of Tony Stark (aside from all of the obvious cool armors).
Comicwise, I'd say that Tony doesn't make an armor completely out of adamantium is because:
1) it's rare
2) it's heavy
3) a denser, and heavier armor would place a huge drain on his power supply
Maybe he could lace the surface of the armor with adamantium, but again, with the magnetic forefield already surrounding the armor, it's overkill.