RAID is a much better use of these disks. You could setup 4 or all 6 of the disks in RAID10 and get better performance and redundancy - if one drive fails, you don't need to restore from backups, you just need to replace the disk. The server and everything on it keeps running the whole time.
However, RAID is not a backup. It won't help if you delete a file. It won't help if multiple drives fail at the same time, or before you finish replacing the first failure.
Backups cost money. Losing all your data usually costs more money. Especially if this server runs services for clients - if you lose all your client data, you will probably lose all your clients as well.
Backups need to be stored separate from the server. The further away the better. Backups in the same machine can be destroyed by anything going wrong with that server - a faulty power supply frying the disks, a drive controller card going haywire and corrupting everything, etc.
Backups stored in the same room or building are better, but can be destroyed by flood, fire, etc.
Backups stored in an off site location are best - both your server and the site with the backups need to be damaged for you to lose your data. For really super critical data, you might even want multiple offsite backups in different locations.
You also need to test your backups periodically - to ensure they work, and ensure you know how to restore them. I'm assuming you don't want to stop your production server for a day to test those backups, so you need them off that machine and onto a test one.
If you think backups cost too much, consider these points:
- When your drives die, what will it cost you to try to recover the data from the failed disks?
- When that fails, what will it cost you in time, effort and money to try to rebuild the server from scratch?
- What will it cost your customers or your business having to deal with the server being down for an extended period of time
- What will it cost your customers or your business to recreate all the lost data? Is it even possible to recreate the lost data?
- What will it cost you in terms of reputation - how many customers will you lose over this?
Those costs all add up very quickly. Backups are a deal in comparison.