value of a HDD sector
> What are the different elements of data I have to protect – how much redundancy is there likely to be between one backup and another, how much does my data change?
That is surly key; by deduplicating data you are making a single section of disk represent a critical part of not just one file but 10s or 1,000s of them. If that sector gets corrupted you will loose not one part of one file but one part of your thousands of files. Of course a properly administered raid (make sure you check the logs!) should mitigate this problem.
Better make sure your monthly off-site backup is not getting deduplicated against last month too if you really want it to be a full copy of all your data. However I do agree that on the whole it should save lots of disk space, which will save capital and energy :-)