In a worst case scenario, i.e., you have a double disk failure, a system in degraded mode cannot complete reconstruction because of all the noise being made by the second "failing" drive causing reboots before reconstruction is complete.
It cannot complete reconstruction because, in an N-disk system, if one disk fails, you need the information on the remaining N-1 disks to reconstruct what was on the now-dead disk - and if all N-1 of them aren't accessible, you lose.
That is basically what a "double disk failure" is. It does not mean that suddenly you cannot access any of your data.
Err, umm, if more than one of the disks in your RAID array have "failed" to the extent that more than one of them is inaccessible to the filer, you cannot, in fact, access any of your data any more, except for copies on your backup tapes. Maybe the reboot will make enough of the disks accessible that the problem is transient, but that's not guaranteed.
(E.g., to take an extreme example, ripping two drives out of your filer and pounding them to scrap metal with a sledgehammer will certainly render data on them inaccessible. Hard read or write errors on two drives will have the same effect, if a reboot doesn't make the errors go away.)