When a unit can target less things it becomes stronger

[AoAGeneral1]

This would prevent the mostly useless state where rocket soldiers shoot infantry, and would allow micro to improve tactics by having the player decide on which vehicle is more important to hit.

This just negated your entire reason for having E3 shooting tanks. If you are going to micro them to fire at certian vehicles then what is the point if you are making them shoot at targets anyways?

The premise behind infantry and unit control is to learn to have them on seperate keys. StarCraft 2 did a really poor job on this by just binding all your units to one key and A-moving about the map while using mouse cursor adjustments with your units. StarCraft BroodWar promoted seperate key bindings to have your forces move in osculated ways for maximum damage and saves.

The old school CNC games also followed this suit as well with the exception of non limited max squad sizes.

If you negate E3 to ignore none armors the promotion is to have them bound to one key which is a bad demonstration on force control. This isn't a A-move win situation its about how the forces are placed and spread. Position and structure placement is extremely important in RA more so then TD.

[Blackened]

I think a point you're missing in all of this too is that this isn't going to make a weaker player better. It's only going to make stronger players even stronger.

[Sleipnir]

Earlier discussions on the topic have focused specifically on the behaviour when a unit automatically acquires a new target (right now it picks an arbitrary unit within its weapon range). Units would not switch from what they are currently attacking if a different unit enters their range (unless that unit then attacks them).