It's all about replicating an enjoyable experience. Think about it: people play sports games, right? And that's despite the fact that they're horribly pale imitations of real sports. Playing DDO might not be the same as playing in person, but if it's close, it can still be fun. Actually, I think DDO would have a relatively good chance of reproducing the play experience. The mechanics are well defined and suited to computer control, you can keyboard/voice chat with your friends in real time, story and simulation are easily conveyed, etc.

It's not the real thing, but when your friends can't be over for a night of DnD, you might just find DDO more fun than you thought.