Frequency licenses are paid for dearly, to leave them unused is stupid. There is no really reason to leave some of the TPs unused for backup purpose. Backups are provided by backup satellites, not by leaving a few TPs unused. When a satellite fails, often it is totally gone, requiring the backup Satelilte to kick in and replace it. I have yet read once when on satelite had some kind of failure and half of its good TPs were activated to replace the other half bad ones. And even if backup TPs are needed, most satellite actually do have spare TPs, in addition to the 32, built in as redundant TPs for backup already. Agains no need to leave any TPs unused for backup.
So Hemi's question is still valid, why leaving so many TPs unused? The only logical answer is there are no more new HDs to add, but clearly there are many more out there not yet added.