Max-Plunder is defined as a system update that requires massive purchase of Nobility/Xstals to stay competitive. Hence, max-plunder of wallets or failure of kingdom.
It is a good guide but a simple version for the noob; Maintain allies of equal value to the cost of your next land up to 20B. Yes spies without allies do not benefit from the additional power well bought allies provide. But that's a different topic.
More Plunder I know this might sound strange but the troop buildings with more defense give out more plunder than attacking troops
I am guilty of selling away my allies. That being said.... That was before I was in a clan. Selling allies was the only way I could truly make money apart from quests. As it stands now, I have a clan, and I value the Allies I am able to retain... You are correct that they are valuable! I think, until you're in the deep end, the shallows use them to make cash.