I have a tape drive connected directly to a server, which have a lot of static data. I.e. the data won't change during the backup window.
Does Amanda still need a holding disk?
I would say yes. One important reason for a cache like this is to allow the backup program to consolidate all data it needs to backup and write it to the tape in one continuous operation. This prevents the tape drive from having to start and stop all the time while the backup program is searching for data to write, which in turn is important because it will have to go back a little on the tape to find it's correct position when starting again, a process called shoe shining for obvious reasons, which puts a lot of wear on both the drive and the tape.
Modern drives are told to be much better at this because they slow down before their write buffer runs empty, reducing the shoe shining effect, but I would still try to prevent it wherever possible.