The power bonus was one thing; that gave a chance to legitimate guilds who worked hard but just came to the game late or are faced with a guild they could beat if it weren't for just one vastly overwhelming player (which happens on every server from what I see personally and have heard). However, the durability thing does, as others said, just reward those same dominant guilds who already have guilds full of alts for extra maps, extra coins and extra credits. Now not only can they defend their cities by placing blocking bids from these alt guilds, these alt guilds can be used to cripple their enemies as well.