Protective is good in the AI hands because it is a trait for someone who is not trying to win - merely to survive. It's not much use for a human who's trying to get to victory. How often do your cities actually come under direct attack? The AI will simply wander round them and rip up your improvements to the point where you have no chance of winning if you just sit in citie hoping they'll throw stuff at city garrison units. I find it to be very rare that I see a direct attack on one of my cities, enough that I don't rate city garrison or walls as worth much at all. Castles have too short a life time in most games for any impact there. Drill is worth having, but doesn't appear on the most useful units, and isn't enough to get this trait out of the bottom tier.