memory - Why not use SSD space as RAM?

I am new to the hardware side of things. I run a few machines which have 400GB+ SSD's and 32GB RAM. I have been thinking about going up to 64GB RAM, however, I was thinking, since SSD's are solid-state like RAM, can't my extra space be used as RAM?

If I do this, will the extra RAM (from disk space) be significantly less efficient than DDR3 RAM?

Answer

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.