Why not use SSD space as RAM?

Firstly, RAM is still significantly faster than both your regular 6gb/s SATA or even the newer PCI-e based solutions. RAM is also designed to be written and erased repeatedly, at the cost of volatility. RAM generally doesn't wear out due to regular use - though, of course, it can fail like any component.

While the lifespans of SSDs have gotten much better, SSDs do wear out. They're absolutely brilliant for nonvolatile use, but if you wrote and overwrote NAND (which SSDs contain) like you do RAM, it would wear out.

Both are really optimised for different things, and you're better off having enough RAM (and using SSDs or spinny hard drives for paging out) than compromising on enough RAM for the task.

Two years after the question was posed, the answer is changing from no to maybe.

Samsung SM951 is the current state of the art and, in RAID 0, has been shown in testing to achieve 4.5GB/s read and 3GB/s write. At a cost of $1/GB per disk this is significantly cheaper than RAM.

http://www.legitreviews.com/samsung-sm951-m2-pcie-ssds-raid0-performance_161753/5

DDR4 data transfer rate:
DDR4 2133：17 GB/s
DDR4 2400：19.2 GB/s
DDR4 2666：21.3 GB/s
DDR4 3200：25.6 GB/s

http://www.transcend-info.com/Support/FAQ-292

Further, the short lifespans of SSDs have been greatly exaggerated with tests showing that the 250GB Samsung 840 Pro sustains 2.4PB of writes.

http://techreport.com/review/27909/the-ssd-endurance-experiment-theyre-all-dead/4

Depends on the application. If speed is more important than space then RAM, otherwise (maybe) look at SSD.

As grawity suggested you already have the swap/page file performing this task. Now even a SSD is much more slower compared to DDR3. SSDs can deliver up to about 654MB/s while 1333MHz DDR3 in dual-channel mode can deliver up to 21.3GB/s (21 332MB/s).