It's true that there are plugins which offer this feature, but even with those measures I still sometimes make a booboo. And it can hurt my opponent as much as it can hurt me - let me offer you an example. Just today I was sweeping my last opponent on Australia, only one territory left. I use all my armies until they are down to 1's, and can't finish him off. So I end the turn and wait for next turn. Whoops! I didn't notice that I had 9 armies left on a connected territory I forgot about. Sucks for my opponent more than it sucks for me, since he has to begrudgingly take one more turn.
That said, I dont actually think there is a pressing need or good argument for an undo button. I just thought it was an interesting example and wanted to share.
