Cons:

1. Um... lore?

(I guess there might be the odd person out there who actually cares about this... and SE used it as a total BS excuse, so might as well put it in the list).

2. Technical Modifications

(Stuff like mounts and PvP rewards... only, this is an easy fix. They simply remove the GC requirement to the wins for each mount (similar to ADS) and reset people's personal win counts for the new challenge. It would suck, certainly, and I'm sure people would gripe, but keep in mind this is not the fist time this has been done. When the ADS mount first came out, it was a similar situation. Some people already had 200 wins, but they did not apply retrospectively. SE did this to us once. They can do it again. Alternatively, they could simply have that challenge redeemable at your GC HQ instead of that old guy in Gridania. That way, the challenge is linked to your GC regardless of wins. If you want a different GC pony, you'd have to swap and start over...which is pretty much the way it is now.)

3. Losing your cross server GC regulars.

(This is probably one of the only objections to removing GC's that I've actually taken seriously. If GC's are removed, you could party up with friends regardless of GC affiliation; however, you would not be able to form a Pre-made party with friends you've made, or regular PvP'ers you've come to respect, who you have met only because you share a GC connection but are on different worlds. That is not to say that you would never meet these people again. You may luck out and find them in PvP, as the non-GC version of the game would still collect players from across worlds. However, there would be no guarantee that they would be on your team. You could now find yourselves enemies. This is a bigger issue for those of us on worlds in which PvP is not active or liked, because Pre-mades would be harder to come by, and the odds of any of those regular players being on your world is remote. That being said, how big of a con this is depends on how you weigh it against que times and the ability to que with friends.)

4. Possible one sided competitions.

(The final con that might be worth considering. Basically, it acknowledges the possibility that, with the removal of GC's, the regular PvP'ers and proliferation of Pre-made groups could create uneven and possibly straight up unfair matches against PUG groups. This is unknowable until it's actually seen in practice though. IF it did happen, it would be a bigger issue for smaller worlds with less dedicated PvP'ers, as they would have less players to pick from to create their own Pre-mades to level the playing field, and would still be outnumbered by the larger worlds with a more dedicated PvP base. Again, these things are impossible to know for sure until it's seen in practice, but it's a fair concern. Also, like #3., this objection is also being weighted against the dramatic increase in que times and the ability to que with friends. Take what you will from it.)