With spanning/concatenation, it's the same problem - you have a logical device that is made up of one or more, except that unlike striping (where at least you get a performance benefit), you still lose the contents of one and you lose the contents of the other as well.
Unlike striping, where data is written across disks in round-robin style, spanning will fill the first disk first, then moving on to the others, filling them each in turn. But your OS still sees it as the same single device, so in the case of Windows would likely write the MFT to the beginning of the first disk. And when that goes, how do you see your data on the second?
If you must join two or more disks together, and you're happy with the pitfalls, then use striping. Forget concat/span.
Hopefully, as an attempt to put a final nail in the coffin of "mine has never failed, therefore it's completely safe", I'd advise those who still think that to pick their favourite hard drive manufacturer, go to their website and search for either "MTBF" or "mean time between failure". This figure is rated in hours, and is largely a guesstimate as to what the life expectancy of a drive might be. Sadly, such a figure isn't a lot of compensation if your shiny new 400gb goes down the toilet in a matter of months, but the point is the manufacturers acknowledge that their product has a life expectancy.
Hit google with "mtbf hard drive" also.
Nuff said.