I think this link explains the reason for this.
"raid power failure dram data backup"
http://www.vikingmodular.com/products/arxcis.asp
ArxCis-NV™ - Non-Volatile DIMM Module
Viking Modular Solutions’ ArxCis-NV™ was designed and developed to securely backup mission critical data in the event of a power failure. This data, which resides in volatile DRAM, historically has been reliant on sub-optimal and unreliable batteries for backup. ArxCis-NV now offers a highly reliable, secure, maintenance-free and disaster-free solution for critical RAID solutions. ArxCis-NV is also environmentally conscious, a “safe and green” product with no toxic battery disposal required.
Loosing a slot is bad but gaining 256gb SSD onto the ram can likely end up as extended ram / swap / pagefile space hardware wise not software that would help a lot of heavy duty video editing / rendering guys for very cheap.
Imagine buy 256GB Dram3 and trying to find a mboard that would support it or this for $800 including DDR3 ram and having Ram+256GB extended SSD ram or more than 1 SSD SATA into each ram module available.
Next thing is to get crossfire / SLi type link from the Dram sata to a pcie 1500GB 750MB/s SSD OCZ card just for better speeds.
I sure as heck would like to try and fill the extended "SSD" ram up during a few big renders.
Might even help with Seti@Home or heavy data compression where huge ammounts of ram is needed and offloaded onto the SSD fake ram.