One of the benefits of a RAID1 array is that the array can detect and automatically correct errors that occur in individual disks at the sector level. When one disk develops a bad sector, the sector is remapped and the good data is copied from the other disk. Even if the sector is not explicitly bad, inconsistencies in the data that was written can be detected and corrected. ZFS does this by "resilvering", other RAID cards like LSI or Adaptec do it by "patrol read" and "consistency checking".
This helps you avoid small amounts of corruption - a byte here, a byte there - from growing on your disk over time.
Do you get these same benefits when using Microsoft Windows 7's Dynamic Disks feature to create a two disk mirror in Disk Management?