My recommendation remains the same: changing backup products is a bad
thing unless you're doing it to meet requirements.  Changing due to cost
of licensing is a really bad idea and won't save you money, will
probably cost you money, and will result in instability and a lot of
pain.

I will two of the comments in Stuart's excellent response:

>Networker ... doesn't ... support the idea of staging to disk

The versions you used didn't have it, but current versions do support
disk staging.  Backups sent to a disk staging device will be
automatically copied to tape to make room for the next set of backups,
and will be kept on disk until space is needed.

>I don't think Networker has anything analogous to TSM's scratch
>pool 

True, but you can tell it to reclaim from other pools.  Meaning if pool
A is out of tapes, and pool B has tapes, it will grab a tape from pool B
and use it.