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.
i5-3570K, ASRock Z77 Pro4-m, Asus GTX660 TOP, 120 GB Vertex 3 Max IOPS, 2 TB Samsung EcoGreen F4, 8GB G-Skill @1.25V, Silverstone PS07B