Offensive is arguably the best all round national idea militarily speaking. The general bonuses effectively give you +1 to rolls, you get nice manpower and forcelimit bonuses, there's the always wanted discipline, and best of all is forced march and the flexibility it brings. That said, do I use forced march that regularly? Not really. The flexibility it brings is just generally useful though. I'm a defensive player myself, but against the AI I tend to try to lure the AI into attacking a stack they outnumber in mountains, only to use forced march on some adjacent stacks to reinforce them and then hunt them down when I win.