A few positives from the tip of the tongue are:
1. Players who crave a more organized setting would finally get to put their ideas and teamwork together in a different gametype.
2. Friends, guilds, and alliances have the chance to play with each other and include more of the same in a match.
3. A new metagame is likley to appear with (semi-)organized teams. Imagine a team in FA spec'd specifically to defend and push amber VS one dead set on breaking through bonds, spirits, wards, weapon spells and the like to assassinate Gunther. Or a Kurzick team so overloaded with offense that they catch an invading Luxons squad offguard, forcing a change in strategy and playstyle.
Also, split possibilities would make for crazy Alliance battles where everyone has to react to all the different maneuvors of the other team.
4. New things are usually fun for a while if for no other reason than they are simply new.
Obvious negatives to this would be:
1. Many players like the random team matching as it eliminates the stress and time of finding and organizing a group. I also feel this way at times.
2. If any actual map imbalances exist they would come to light, be exploited, and ruin the enjoyment of the other teams, especially if there's no way to overcome or outplay this advantage with tactics, or player skill.
3. Party wide skills like triple-aegis, heal party with hb, light of deliverance(?) etc. would need hard counters. To a lesser extent so would spirits and minions. Spikes are minor as they can be beaten with smart splitting. Soul Reaping, however, would be a MAJOR issue

These are just a few things from the top of my head. I'd really like to hear what the rest of the community thinks and what other positive or negative issues you think may pop up.