First of all, DDR RAM is not cheap (at least, not compared to NAND RAM). It costs significantly more per gigabyte than even the most expensive of Intel's offerings for SSD's. While it should provide more theoretical throughput than any SSD, benchmarks at various places (http://techreport.com/articles.x/16255/1) haven't shown that to be significant yet, at least from the end user perspective (some synthetic benchmarks show that the RAM based disks can be faster than SSD's, but translating that to real world usage scenarios by consumers doesn't quite show any tangible benefits).
DDR RAM uses up a very large amount of power per stick compared to SSD's do. I remember seeing the power consumption of one of the DDR2 based "volatile hard drives", and it was higher than spinning drives (at least at idle), and wasn't particularly faster than the best of intel's SSD's.
So sounds like DDR RAM on board is expensive, power hungry, and doesn't provide that much of a tangible benefit to consumers. Tell me again why it's a good idea?