You beat me to it Liam: I saw the way things were going on the Solee thread and KNEW we needed some space for debating the Guild. Here goes.
The Guild, taken in the context of its original mandate/raison d'etre, deserves to exist becasue it meets legitimate needs in a legitmate manner. Anyone familiar with Hard Times is familiar with the devestating effects of localization on the worlds of the Imperium. Specialized worlds, dependent on stellar commerce and communication, ceased to exist or were often forced to fall back on subsistence level micro-economics: a pretty way of saying they descended into savagery if they had a viable biosphere. Independant worlds, capable of sustaining themselves, STILL suffered because they represented tempting targets for the potential conqueror: they faced (often irreparable) damage to their infrastructures if they resisted invasion, they saw the fruits of their labours dissapear into space if they surrendered, and, if they were really unlucky, looked forward to anihilation because the contestants KNEW they couldn't hold the world yet wanted to deny it to the enemy.
What do these problems have in common?
Interstellar government.
In interstellar polities such as the Imperium, Solomani Confederation and their successors, trade and communications are dependant on the state of the government. In such a polity the trade routes must be policed, regulated and taxed: remove or damage the governing body and you remove or damage the route and everything it represents (at least trade and communications, at most civilization itself.) The Guild stands for the routes themselves, with none of the entagelments associated with interstellar government: they will maintain the flow of goods and ideas while the individual world is sovereign unto itself. They will not police you, they will not tax you, and they will not bomb you from orbit because they are not a government. Each world trades on its own terms for what it wants and needs. Each world maintains the fleet it wants and needs. But that is the business of each world. The business of the Guild is just that: business.
Interstellar civilization does not need interstellar government: there are plenty of historical precedents for distinct civilizations composed of a multiplicity of independant (sometimes to the point of warring) governments: each looking after itself. The Greek city-states and classical civilization in general are a success story born of the same principles. The new TV show "Firely" showcases a universe experiencing the tensions between stellar and interstellar government: why shouldn't every system be a power unto itself?
Some of you have been sitting at your computers saying (out loud): "But the Guild is BAD! Really, really BAD! It says so in the Book!" Well yes, since that palace coup and the new direction it took the Guild does practice extortion and slavery. I quite agree, those things are BAD. However, these are recent events and the Guild still represents enormous potential IF it can be redirected toward its original purpose (a campaign itself: player-traders stage a takeover). You don't burn a house down if it get infested: you call the exterminator. Likewise, the Guild can be saved in canon or your own private campain universe.
Won't they form a monopoly? Not if they want to stay in business: individual members will compete with each other and individual worlds will form independant trade fleets to offset monopoly-minded magnates.
Why would interstellar goverments themselves keep them around? Because they perform an essential service, freeing up talent and capital for the government to pursue other projects.
Won't worlds turn inward on themselves? Not necessarily. The flow of ideas and goods is the lifeblood of any civilization: if you don't want to trade and communicate, that's your choice but an interstellar government can't prevent that any more than the Guild can.
There you go. I've done my long-winded devil's advocate bit for the Guild. Let's hear from someone else.