When you decide whether to do a full, incremental, or incremental_since_full backup, you need to balance the time it takes to create the backup with the time it would take to restore. You also should consider media requirements. A given incremental backup is relatively quick and takes a relatively small amount of space on tape or disk. Full backups are relatively slow and require a lot of space.
Incremental_since_full is somewhere in between. It starts out as equivalent to incremental, but as the database changes and the number of backups since a full backup increases, incremental_since_full can become as time-consuming and media-consuming as a full backup, or worse.
In general, the opposite is true for restore operations. For example, if you need to restore from a very old full backup and a dozen or more incrementals, the restore may take longer and the backup may use up more space than a new full backup.
The obvious advantage of incremental backups is that it is much faster and takes less space to back up only the data that has changed since the last backup, or even since the last full backup, than to back up your entire database. The disadvantage of relying too heavily on incremental backups is that any eventual restore takes longer.
For example, once you have a full backup of your database, in theory you could perform only incremental backups thereafter. You would not want to do this, however, because any future recovery would be intolerably slow, and would require more tape or disk space than doing a full backup periodically. Remember that other users can have read and write access while you do backups, but no one else can use the database while you are restoring it. You might find yourself needing to restore dozens of incremental backups, with your system unavailable to users throughout the process.
A much better approach is a mix of incremental and full backups.
The greater the volume of your database changes, the more important it is do a backup, and the smaller the advantage of incremental backups. For example, if you update your database nightly with changes that affect 10 percent or more of the data, you may want to do an incremental_since_full backup each night, and a full backup once a week. On the other hand, if your changes tend to be few, a full backup once a month with incrementals in between might be fine.
