No, mSata is not limited to SATA II. mSata is purely a form factor, like 2.5" drives. The connector is identical. mSata drives tend to be a few dollars more due to lower demand.
The main reason to go mSata is what you said, though: it's the only feasible way to get two or more drives into a laptop. mSata for OS and programs and hard drive in the 2.5" bay for bulk data is much less constraining than SSD only.
If you're nuts, you can get a laptop with 2 mSata slots and configure them as Raid 0. I've seen at least one guy talk about doing exactly that and getting 1000 MB/S in Crystal Disk Info.