Are you talking about something like Striped+Mirroring (RAID 0+1)?
Or are you talking about having several RAID boxes providing say RAID 5, and then joining them via some other RAID device into say a Mirrored volume?
Striped+Mirrored is a way to get redundant high transfer rate storage, assuming you have a fast enough channel connected between the Mac and the device (eg SAN, or all internal devices, or dedicated RAID card with multiple device ports, etc...). This Striped+Mirrored is frequently used in high performance computing.
If you are NOT talking about Striped+Mirrored, then it is less likely to offer much benefit. The forum participants would have to know exactly what you had in mind to give an opinion.