1) From previous experience on several games, faction based pvp is terrible, unbalanced and unenjoyable.
2) You're excluded to join your friends if they're in different GCs so you're bound to pug with random people and most likely get doomed to fail (lack of communication). Don't tell me you can change GCs because that's not a rational solution. Some people have cooldown on the GC switching, others might be doing certain GC achievements, others might lose their pvp ranks (following someone who made calcs on forums about it, would take nearly a year?).
3) For the above reasons, people will probably spam frontlines after update because it's something new, and will be ignored some time after that because people will be bored of losing and not enjoying frontlines while getting stomped by premades who were able to switch GC or already were in the same.

They should lower the cd to swtich GC drastically at least.