It isn't a recipe for disaster, it is what you need if you have large amounts of data. Or a very, very large iTunes library.
First off, a pair of HDs in RAID 0 can saturate the SATA II bus, which is the limiting factor in an older Mac Pro, unless you go with a SATA III RAID controller card - not recommended (cost vs performance) - so it certainly is faster. Secondly, some of us need that much space. I have 6 HDs in 3 RAID 0s. (see sig). No muss, no fuss.
Now, it is important to have a proper, verified working backup strategy if you go this route. In my case, I have a Qx2 with 4x2TB in a RAID 5. By the end of the year, I'll be looking to replace the 2TBs with 4TBs (and getting another Qx2).