And if you're playing broodwar, then you can have cloaked siege tanks
![devil.gif](http://www.chefelf.com/forums/public/style_emoticons/default/devil.gif)
As for the A.I. Blizzard games have notoriously vicious AI in skirmish mode (warcraft 3's a bit of an exception). It's always geared up for the short game, in a similar way to most online AOE matches. Most of the time you factor in an action delay when coding A.I. to reduce the computers reaction time and make things more balanced.