Are Player Factions Worthwhile?

The guys at Rift Watchers are starting an RP-PVP guild when the game finally makes its debut.  Personally, I've never been that fond of the PVP ruleset, but I might be willing to give it a try - perhaps pressure to make myself a less attractive target to gankers would encourage me to group more.  There's only one minor problem.  They're rolling Defiant, and my limited experiences so far in game are telling me that I'd rather roll Guardian. 

Building the lore....
In an interview with the lore leads for the game's two factions, Trion told the RW crew that the best faction is the one that each individual player enjoys the most. 

I tried Defiant first, and wasn't that impressed.  The whole "look at us, we're rebellious and challenge authority" attitude feels cliched, and the faction's introductory storyline features a contrived time travel plot that doesn't appear to explain how the Defiant can have any player characters who weren't physically present with the player when they personally hit level 6 (since the portal to the past appears to be open for just long enough for the player's party to jump through).

In fairness, the Guardian story has its share of fantasy cliches as well.  Somehow, I just preferred the feel of the faction, which I suppose is precisely what Trion's lore team worked so hard to accomplish.  Players should not only identify with their faction but actually feel a stake in beating the other side - and not just because they happen to be sitting on some in-game bonus that players would like to capture for themselves.  That said, has Trion done the job of making the factions stand apart a little too well?   

...by separating people?
EQ2 nominally has two factions, and I'm sure that some people really care about the respective lore, but I'll concede that I don't really care.  Players are free to group and guild with characters from the other side, and it's left to the individual to decide whether and how to address the good versus evil conflict.   As long as you're on the correct server (and it isn't the game's sole surviving PVP server), you can play with your friends, even if your character races are incompatible in the lore. 

By contrast, a hard split between factions, as in WoW, Warhammer, or Rift, does the game no favors when it comes to gameplay.  WoW in particular hasn't done much to justify segregating the playerbase.  Unlike Warhammer, open PVP between factions was not a major factor in Azeroth in recent years.  WoW's last two expansions featured neutral cities, neutral factions, and enemies that both of the two warring factions wanted dead.  The flavor text was different, but it often felt that there was no good reason why we shouldn't be able to join our friends on a raid against common foes - indeed, the two sides attempted just that during the famous Wrathgate cinematic, straight from Blizzard. 

To the extent that Trion succeeds in making the faction lore matter, they add yet another factor - Defiant or Guardian - to servers and levels and itemization as reasons why players in a supposedly social genre are prevented from playing with their friends.  If they don't succeed, though, they incur all the costs of having two factions without the benefit of a more immersive world.  Right now, it's feeling like a lose/lose situation.  I wonder whether Trion will wish in the long run that they'd gone with a more permissive lore.