with RAID 1, the disks are used symetrically (i.e. they're used at the same rate for all transactions). every time you write to one of the drives in the array, you're writing to all of them. so they all get the EXACT same usage. which means the probability of all of the drives in the array failing at the same time is increased (of course this is only failure due to usage/wear and tear, not manufacturer errors or other failures) . also, in a RAID 1 array, the total size of the array is only as large as the smallest disk in the array (i.e. if you've got a RAID1 array with 3 x 1tb drives in it, your total usable size will be 1tb.)
something else to note about RAID 1
While RAID 1 can be an effective protection against physical disk failure, it does not provide protection against data corruption due to viruses, accidental file changes or deletions, or any other data-specific changes. By design, any such changes will be instantly mirrored to every drive in the array segment. A virus, for example, that damages data on one drive in a RAID 1 array will damage the same data on all other drives in the array at the same time. For this reason systems using RAID 1 to protect against physical drive failure should also have a traditional data backup process in place to allow data restoration to previous points in time. It would seem self-evident that any system critical enough to require disk redundancy also needs the protection of reliable data backups.
RAID 5 offers a bit more security against some of this stuff (granted, something like RAID 6, 10 or 50 could be better)