This is my take on the microcast piston,
First, most pistons are cast oversize then CNC ground to shape on the outside to achieve the familiar finish. Not CNC'd from billet, as that doesn't give decent performance.
Most cast pistons however have a lower silicon content, as it is very difficult to achieve the ultra high silicon percentages using a std casting method (the blending of silicon to aluminum becomes very difficult above 22% as the silicon doesn't want to blend after this ratio, and requires very costly techniques to force the silicon to bond to the aluminum).
Microcast pistons on the otherhand, begin as a powder with the high silicon content dispersed as a powder with the aluminum powder along with a bonding agent(this allows the silicon molecules to bond properly to the aluminum molecules under extreme pressure). This mixture of powder is poured into the mold and compressed under extreme force and heat to fuse the molecules together to make a piston (in this case microcast could be compared to forging). The benefits to this process is the piston contains a high silicon content (probably around 26-27%) and the piston performs quite well. The downside to this process is the pistons dont last very long.
I'm sure the companies that use this process arent trying to build a product that only lasts a short time, but they are trying to cut costs of production. While truly casting pistons with high silicon content is very expensive, microcasting the same silicon content can be done much cheaper.

I may be wrong on some or all of this, but this is how it was explained to me. Take it with a grain of salt.

BK