Commander weak? certainly not, it is actually one of the best class if you already play the game hundred times.
If you are initiative focused, constant spam battlecry is even better than other class except mage that spam blizzard, a fireball mage is losing this contest easily against commander with battlecry. the gap is even further if the commander is leading a ranged heavy units (especially mage units, i say units, not champion).
If you have henchmen, due to henchmen champion doesn't split exp, you can have 3 henchmen commanders leading entire army full of mages in no time, all kind of enemy army will never act and will be slaughtered very fast due to battle cry spam.
In late game, if you have strong army technology, commander easily tops out other class, yes, even against mage with blizzard.
Its capability to make road is also a very strong bonus, both for defensive or offensive purpose.
Its exp bonus as noted by others above, is insane bonus, especially if your commander is a henchmen, because he won't drain exp, meaning no exp split.
However if you use commander for administrative purpose (meaning you take traits that boost research/gildar/unrest reduction), you certainly use commander in a wrong way. But i agree if a rework is needed for this administrative trait, especially the gildar and research one.