Guilds are allowed now because Blizzard removed the XP Bonus guild perk. Being a part of a guild gives you only two significant advantages now:
Hearthstone is now 15 minutes and Riding speed is increased by 10%. While these do give you an advantage over a non-guilded challenger, the
social benefits FAR OUTWEIGH being on your own.
Please note that you are not allowed to use the Guild Repair option as this is giving you gold you wouldn’t normally have access to on a
solo character. The guild is supposed to be social only.