I usually play with raging barbs, and I like to build the great wall, because of the great spy though. It hardly comes to use for blocking barbs, because I play on small maps with lots of civs
I like to have the islands and/or unsettled continents crowded with raging barbs.
I think it's a cheat only in the sense that deliberately choosing any settings gives the human player an advantage over the AI -- including choosing the civs and leaders in the game, and especially the one that the human player uses.
I always use raging barbs...I NEVER build the great wall, got other more importanmt stuff to build. If a guy can't overcome the raging barbs it is quite likely you will be unable to beat the AIs in that game. If you can waste the barbs you for sure you should have a pretty strong army...well promoted.
Also -- I sometimes pick raging barbs, but I always add more civs than the default map calls for (e.g., 7 on "small", 8-10 on "standard"), so the barbs generally have less free space, and a smaller window in which to generate.
I almost always have raging barbarians on, but not in order to build the great wall and have them attack my opponents--I'd much rather have my early axemen out there fighting them to get the experience. (I tend to keep my axemen the whole game, upgrading them several times. Which I can afford to do, because I have chosen a financial leader and spammed cottages all over!)
I find that setting Raging Barbs and then building the Great Wall can be a big detriment. On most of the games I've played (I always play w/ 18 civs) maybe 1 or 2 of my rivals will have Barb problems. The rest will wind up with multiple, highly promoted archers, making any attempt at early warfare very difficult.
when i want the world to stay rather peaceful till late game i check raging barbs. this seems to keep the AI's busy so there is less war till late game. works well in the next war epic mod. otherwise you get wars early on and there really is no full scale wars on late game with high tech weapons.
I almost always play with no barbs, don't want to be bothered in early game when i want to focus on expansion&early wonder (I often try to get SH or Oracle depending on whether i have early stone or marble, even if i lose the race i will get decent amount of money which is very useful for early game), I also rarely build GW, i prefer SH much more.
i aim for "build the great wall at all costs" when i'm playing OCC and going for a conquest victory. with raging barbs, they're gonna spawn and keep spawning since the map gets more and more fog of war as the AI civs die off. i have to go kill those guys, i don't want to waste effort protecting my land from barb pillaging! the extra GGs are nice but that's not why i do it.
I play with raging barbarians just like I played with the active wildlife or what ever it was called in Alpha Centari, but I don't make it a key to my strategy to build the wall. I played a game with standard barb activity, no wall, and had big problems because it was an oasis map with hard to settle land. I have it on in my current game but it's one big island so there isn't any open land to spawn.
Although you're probably the only one who knows to go get the wall to take advantage of it, someone might still beat you to it, or barbarians might still not be a large issue, and maybe some other wonder will become the key factor in the game and your wall just becomes decoration.