In my current game i am playing Gilgamesh. I used vultures to rush Charlemagne(protective) and pretty much butchered him. It is a huge tectonics map and all but one of his cities was in a nice position. So I kept them. My economy CRASHED.

then i did some wall whip/chops w/stone. I raked in mucho casho. Ran at a huge deficit while teching myst/poly/priesthood/IW. I went for IW rather than an economic tech because I have a large area sealed off to my NE that had gems and lots of jungles.
Besides the wall whip. Protective has the advantage of being able to build a cheap wall and stout archers. If you have REX'ed til bankruptcy sometimes you cannot afford to maintain a strong standing army to defeat enemy SODs in the open field. At that point the timer starts ticking. You have until the AI gets construction (cats) to get your finances in order. Until siege rolls along 2-3 protective archers behind walls will stand a lot of abuse. It is cheaper to maintain 2-3 archers per border city than to maintain multiple defensive anti-sod stacks. One for each end of your empire. When the DOW occurs you have maybe 2-3 turns to get your troops where they need to be. Unless you have a small circle shaped empire, stationing your anti SOD stack in the center is not good enough. So you need more than one. Especially if you get dogpiled. If you get dog piled you then need to decide which SOD to take out and which SOD is allowed to take a city.
Consider a 6 city empire with 4 of those cities on the border. You need 6 units regardless just to keep them from whining about no military protection. So that means 4 extra archers. Cheap to build and with 6 cities I think they don't cost anything to maintain. Assuming just 1 happiness modifier (a religion, gold, ivory, gems, silver or fur) you can have pop5 cities. OK, pop4 most of the time due to whipping. Keep a single hammer invested in an unbuilt archer and 1 hammer in an unbuilt spear/axe at all times. I usually do this when I am regrowing from a whip and am working 0 hammer food tiles.Even in a worst case scenario where the enemy crosses the border and is two turns from attacking the city, you can 1 pop whip the archer with 6:hammers overflow to 1 pop whip the spear/axe. In 2 turns you now have 4 defenders behind walls. Without siege it takes a pretty big SOD of axes/swords to take that city. Ask yourself how many units you would need to take a city with 50% defense and 3CG2D1 archers (2 of which are fortified for the full 25%) and one unpromoted axe? We are talking about an early war so the enemy probably doesn't have CR2 units. (no settled GG, no vassalage/ theocracy). The early war mongers like ragnar, alex, monty, shaka and Ghegis are also AGG leaders. So your shock axes are also facing shock axes. Plus the additional cost of building spears for stack defense. And for city defense. If you are defending your cities with just axes then you're asking to be taken out by chariots. Even with one axe one spearman you have inadequete defenses. against enemy axes/swords the spear is useless. Against enemy chariots the the axe is useless and you have less time to whip an extra spear.
Assuming axes and spears are your prefered method of defense. The 25

saved on walls is the nearly the cost of an extra axe or spear. Pre-construction walls are more under-rated than the Protective trait. Especially if you have REX'ed hard and are tight on the gold to pay for additional troops. Walls are FREE to maintain. Since you cannot afford the troops to keep your power numbers up, even the tiny(and I admit it is tiny) little boost to your power rating from all those walls might be enough to keep you from getting DOW. But if you have a loony neighbor you are probably getting DOW'ed anyway. Your economy is crushed 0-10% and building more troops is NOT an option. Walls can help you survive long enough to tech/bulb/steal your way out of financial ruin. No matter what your choice of city defender is, Walls make then much tougher to kill.
If you can afford the extra units to mount a counter attack then you haven't REX'ed hard enough. Build more units and go take more cities for the conquest gold. But if you have grabbed as much land as possible then you are broke and better off paying the unit costs on workers to get those cottages built, farms built or mines built. You just have to survive long enough for your superior land to vault you ahead. Walls can do that. Sometimes leveraging a trait just means not dying rather than getting ahead.
I am a confessed cowardly, turtling little builder type. I rex hard then hide in my little cities hoping noone notices me while I recover. When they do notice, I rarely (as in just about never) lose a city. Protective archers and walls are cheap. Pre-construction they will kill hordes of attackers. And the ones that die are easily replaced. A CG2D1 archer in a city behind walls usually kills 2-3 attackers before he dies. IF he dies. And then I have traded 25 hammers for 105 hammers the enemy. If one kills 2 attakers and lives then he is CG3D1....in a city...behind walls....fortified for 5 turns.....better bring a LOT more troops. Actually please bring more. I like the GG points (especially since i love the Great wall). First GG gets settled and now I am spamming CG3D1 archers....in a city....behind walls....fortified for 5 turns.
When I finally get peace and my economy turns around I am going to be coming after you....with CR2 units right from the barracks without an XP civic....courtesy of the GG that YOU gave me.