There is no "Lore" reason for the draconian Grand Company restriction.
I'm usually the first person to say I love the Grand Company aspect of PvP, but there is absolutely no reason they couldn't have implemented a Mercenary system by now... Let people who don't care queue as a Mercenary and have them dropped in any Grand Company. Have parties queue up as the leaders Grand Company. All they need to do is justify it in "Lore" by saying its to ensure fair and faster matches. Done.
Still the same Maelstrong verses Win Adder atmosphere with less of the queue problems. People can hate on the "Maelstrong" stuff all they like, but it makes PvP a lot more welcoming if you're automatically part of a static team, rather than it just being the sopro statics verses losers.
