You back to disk because it's typically faster than tape, and restores are better than getting it from tape.
Tape still has a place, and that is for taking your backups off-site. So you can have a policy that backsup to disk using the Dedupe option, and when that job is done, it makes a copy of the disk backup to tape.
Now you have two copies of your backup.
1. For quick local restore using a significant less amount of space than without Dedupe
2. A remote copy of the backup for DR purposes.
There are a number of companies that have eliminated tape altogether though. With the right dedupe backend, you could have months to years of FULL backups on Disk.