I personally like the idea of an mSATA or M.2 SSD because they offer the same performance, but are tiny in comparison to 2.5" drives, reduce cable clutter, and as you mentioned, free up additional hdd slots. I really want to have one in my next PC. As Emrys pointed out, the downside being they won't be overly "recyclable" into other systems/enclosures. But that's a minor drawback in my book.
Main: i5-3570K, ASRock Z77 Pro4-M, MSI RX480 8G, 500GB Crucial BX100, 2 TB Samsung EcoGreen F4, 16GB 1600MHz G.Skill @1.25V, EVGA 550-G2, Silverstone PS07B
HTPC: A8-5600K, MSI FM2-A75IA-E53, 4TB Seagate SSHD, 8GB 1866MHz G.Skill, Crosley D-25 Case Mod