I have been neutral for about 3 months both before and after the OP patch. I have heard a lot the preaching about the supposed advantages you have from being faction. One by one all the supposed advantages were shattered by experience. That's being said in a pacific life where we have guild hunts, solo exploring and levelling, not much pvp. People told me we will not have PR tps. Who needs PR before they get 150 digging? Who needs PR tps before being decently developped in underground digging? I was told best levelling places can be reached only by faction tps: I levelled 250 ele with no need of such tps. I constantly refused to promise I'll go kara or kami to anyone untill decided. That was never an impediment in making friends out of the people I wish they are my friends.
We didn't have an OP at the time but I am sure that friendship prevails whatever your faction is. And I can provide a concrete list of examples where kami defended kara and vice versa.
List may continue...
Finaly the storyline for me personally has a flaw: Factions, in the kara-kami layer, imply religious war. Or I personally hate so much this concept that I cannot adhere at it even in a game. Religious war was and still is maybe the biggest plague we still didn't find any vaccine for. Only by finding a slightly personal different story I could bring myself to be part of it. Also I can say the Kami lore is somewhat flexible enough to alow me to create this different personal story.
If you think you cannot trust a neutral because he is not your faction then you have a big problem. Trust is not based on game labels but in knowing the people you are relying upon. You trust friends or you just don't trust anyone.
There is though a noticeable tendency of players to resist to the complexity of the game. Some of them leave because they are never able to make an efficient action, some of them are unhappy because they cannot accept a system that has some 15 faction flavours instead of the classical 1:1.
It's not a sin, it's not to blame, it's just the natural tendency to simplify. But simplifying spoils a lot the fun

So I certainly respect the people who see it all about kara versus kami but I am also sorry a bit that they refuse the huge puzzle Nevrax created for us, maybe the first true simulation of real political life in a MMO.
So personally (not talking for guild, they each chose for different reasons) I chose faction because most of us consider pvp as part of our fun, and faction certainly gives some advantages (flags, open pvp, tps, chat channel etc), because I could pick from the rich storyline something convenient as to avoid fanatism, and finally because to those friends who prefer imagining things in black and white, I thought it will be boring to strive proving our faithfullness each time they question it
The three months we stayed neutral taught me a lot of things and I am grateful we didn't go faction from the first moment. I enjoy being kami a lot but I don't see it as an upgrade from neutral but as an alternative.