The tactical AI has evolved greatly over the years. While it may not be perfect in every scenario, overall it is really quite good, and can offer up a good challenge throughout the course of a game. If all that remains of the enemy are archers, or they have been engaged in melee, they are doomed anyway, so who they attack is pretty much moot. However, if they are attacking units that are less of an overall threat, then you possibly have an issue that should be reported.