Actually, being part of a good guild is worth way more than just upgrading the rarity of the chests. If you're part of a guild which finishes all the tasks each week, you'll receive a tonne of stuff. Specifically, 590 Gems, 42 Event Keys, 360 Glory, 63 Gem Keys, 316 Glory Keys, 5900 Souls, 670 Gold Keys, and 9 Maps. If you want to advance more quickly in this game, being part of a good/active guild is critical. Good luck!