[DISCLAIMER: Create all the alliances you like, I really do not mind, and certainly have no power to make you create one, or stay out of one]
My two copper pieces worth...
In most societies, the "good" join together for protection and safety. This normally results in the creation and documentation of rules and laws to live by.
This is why most societies have more law-abiding citizens than they do criminals.
As time goes by, those laws/rules get more and more complicated, and we end up with a serious infestation of lawyers.
Thank goodness Siege isn't quite at THAT point yet! We don't want a lawyer template!!! (Samlawyers? Lawpires?)
Anyway, while "good" people might find it useful, even helpful, to have those structures, "evil" people generally do not. They see those laws as a tool to oppress them.
One reason for that is, they tend to be out for themselves.
Now, before you all jump down my throat, think about it. A "strong" (in terms of actual power, wealth, OR influence) leader will typically gather people who can further their goals around them. That's not necessarily just an evil thing, it applies to good as well. But, in the case of evil, the goal is normally (not always) to put the leader at the top of the heap.
Alliances between evil groups may occur from time to time, but they will generally dissolve when one leader's ambitions outstrip the others.
But, I would expect to see alliances between evil parties appear and die over time.
However, there is another group, who are considered to be good or evil depending on your perspective, and I WOULD expect to see alliances between these people.
These are your freedom fighters, your rebels - those who are fed up with the current "regime" and want to take power away from the current "rulers", in order to change society for what they see as a better way.
Looking at a LOT of the recent discussions on these forums, I think that some of the people who identify themselves as "dark" actually fall into this last category.
From the point of view of the "white", yes, they are evil. If this was real life, some might even call them terrorists.
From their own point of view, they are not. They are fighting for their rights, and their freedoms. To my mind, that fits well with a lot of what I hear going on around here lately.
Just my observations and opinions, not intended to get anyone worked up, I'm not telling you to change your roleplay, or even to convince Grim to change Sundays to "Rebel Alliance Day". *grins*
I'm just suggesting that a "dark" alliance might not really be the best fit. Or I'm over-analysing things. Anyway, that's my two copper pieces. Don't spend them all in one place!