Which is better, a Toyota Camry or a Whirlpool Front-loading washer?
RAID Mirroring and Backup are two different things entirely. RAID 1 gives you almost up to the instant protection against a single drive failure, but it give you absolutely no history or roll-back ability. If you delete or overwrite a file, or have a crash that damages the file structure, it is deleted/damaged on the mirror at the same time as the primary drive.
TimeMachine is all about rolling back to a previous state (indeed, multiple previous states in history). If you have a primary drive failure, you are likely to lose changes since the last write to the TM drive.