As training for games against human opponents, skirmish is a joke. The AI consistently displays a lack of unit tactics and realistic behavior (Something that was somewhat successfully coded into War3), particularly when massing units (If you can call what the computer does a "mass".) It never sends enough units per wave, and rarely selects the appropriate targets for each unit type.
These deficiencies are especially apparent in the latter missions of Single Player campaigns. Initially, the computer's standing units and preset defences will be difficult to overcome. However once the money starts flowing and you attain a good rate of unit production you become unstoppable, since the computer's counterattacks don't adapt to yours.
"Nothing livens up a robotic hymn of doom more than an amazing pair of jugs." - Brak