At first I quite liked the idea of a simple backup program where I could use my DV cam to backup Mac data, however a little research and the following are (imho) key reasons not to use such a system:

• You would definitely need to us error checking which will reduce the amount of data stored per tape
• Each tape only holds 10GB per hour, prob less with error checking
  • Tapes are expensive
  • Tapes designed for video allow for a bit of error as it is not so important
  • It would take an hour to move 10GB to an hour tape (more with error checking?)
• Recordable DVD's are cheaper than tapes
• DVD takes short time to burn
• New gen DVD's hold twice as much as old ones (Dual layer, no idea about BlueRay)
  
• MOST IMPORTANTLY - with DVD's you aren't locked into some unusual and propriety format that may not be viable in 5 years. DVD's on the other hand can us the basic data formats, can be moved to other machines with little hassle and will hopefully still be accessible in 5 years (Anyone still use floppies?)