While software solutions have had their issues as well, but a solution built right into the OS should eliminate most of those issues while having the added benefit of being right there for the user to take advantage of without having to add or install anything extra.

The ease of customization also makes it more flexible than most RAID solutions, at least for mirroring applications.

Conversely though, RAID provides more options than just mirroring for protecting your data and can support more drive configurations but this solution should be enough for most using portable devices and the usually limited number of drives they can use. 

Portable devices also usually make it more important for data security rather than drive performance, especially for hard drives.  While the increasing use of SSDs helps eliminate the need for drive performance enhancement.