Jetpacks as an armor ability destroyed map control. Did your team manage to beat the enemy team in claiming the higher areas of the map? Did you manage to force them into areas with less vision, less control, and required them to come towards you while you gained the ability to pick them off? The moment they have jetpack as an armor ability that all changes. All that work, nullified, because all it takes is the hold of a button and they can get right up next to you. Cover isn't exactly the issue in my eyes; its the fact that the work you spent on claiming the areas of the map for your team to gain a strategic advantage is absolutely useless because the strategic advantage is negated by an object which players can spawn with.
A jetpack is fine with me if it's a picked-up armor ability, since that, itself, is a tactical advantage that players need to fight for, similar to power weapons, or ovie/camo/equipment in previous games.