The canon doesn't give any indication as to the importance of crystal.  Seeing as crystal costs are highest for technologies and advanced units and structures, we can presume it has something to do with high-tech equipment.

I think somewhere in the lore section it mentions that advent use the crystals as a source of their psionic powers, and one of the reasons they're on their crusade is because their sources of crystal are running dry.

Could be an energy source like dilithium

Me I can easily see crystals as the main componets to technologie and ellectronis. Kinda like in Stargate, The Gouahoul, Asgards, Ancients and the Torri use crystals to power and control all of their advance technologies.

I think it goes back to StarCraft where you mined crystals and gas. The crystals looked pretty on a map, as opposed, say, to piles of elephant poop. The gas geysers blew out attractive green gusts to be visually pleasing also.

Scouts and light frigates cost no crystal probably because low end SC units required no gas, thus giving you a bit of time to get gas mining going and take care of more important matters such as rushing. Here it is initial colonizing and perhaps fending off pirates.

Because how would the Advent strap giant crystals to their Capitals if they didn't have ludicrous quantities of it?

They could be martian marshmallows or venusian vines, or plutonian petit-pois and you would still have to harvest them and pay the resource cost of your stuff. makes no difference. 

The spice controls the universe.

It is by will alone I set my mind in motion.

MUAD'DIB!! MUAD'DIB!! MUAD'DIB!!

that's because the Marza has not been trained in the Bene Gesserit Way. And it's just 'Voice', not 'The Voice'. Trust me, I'm a Dune fanatic.

yes... because Sins is a total rip off of Star Craft.... Fail!

Its crystal because crystal is what is used in advanced electronics and technology. To research things you need lots of crystal (because experimentation is just a fancy way of saying educated trial and error), whereas to build you need less, but you still need it so it can be incorporated into the design, (like testing silicon chips by using silicon then selling the completed silicon chips without silicon...). Crystal also has structures and formations within itself, its like a natural hard drive, if you can map the inside of the crystal, you can then access and write data directly to and from the material of the shard.

by the way, this is total science fiction and in no way, shape or form factual.

Why Crystal? because it sounds Science Fiction-y =P

This.