So the title practically speaks for itself. Some of you might say that I need "a balance" or whatever, but from playing as Isengard, I can tell you that is not a good option. Isengard has a poor economy until you conquer a major moneymaker like Minas Tirith to support your ever growing campaign, so ranged units should be the least invested part of your army in terms of hiring units. The crossbows are AP and have much better stats overall than Uruk Archers, but I find the Uruk Archers hit faster, use flaming arrows, move faster, and don't have the straight line of fire.
I rarely use ranged units when playing orc factions, as they are more of a melee strategy campaign and their archers aren't even decent, but Isengard campaigns usually require a lot of defending (you can't tell me you blitz the entire campaign) and ranged units are extremely valuable in the M2 engine.