Some tarsnap users back up all of their data; others just back up a subset. I'm not sure why you say that snapshotting isn't important -- in addition to the savings in bandwidth and storage, it makes tarsnap much faster and more convenient.
if the tarsnap server ever goes away, customers risk losing their data, since the server maintains the mapping of S3 objects to blobs
If the tarsnap server dies, I can launch a replacement EC2 instance and regenerate the metadata (as described in the article). If you're concerned about the possibility of me going away... well, if I get hit by a bus the tarsnap server will keep running on its own for at least the immediate future, but I have to concede that at the moment tarsnap won't survive indefinitely without me.
But I do my best to make sure that I won't get hit by a bus. :-)
Agreed. I use duplicity instead, which is a Free program that is similar to tarsnap. It backs up to S3, but uses many fewer PUTs (since archives are several megabytes).
Anyway, a full backup of my homedir (minus music) costs me about $1.40 a month to store (with incremental backups every night). A small price to pay knowing that if my laptop blows up, I can be right back where I started in just a few hours. (Or if I delete a file accidentally, it is back in seconds.)
My current strategy is a little lame but works quite well: rsync daily to my home server and about once or twice a week an EC2 instance is fired up with elastic block store attachment and the home server does rdiff-backup to it.
Or maybe you're just doing a subset of your data, in which case I don't see how snapshotting is such a big win. Nice to have, but not important in the least if you are just doing a small subset of your data. True, it saves a few pennies on transfer charges, but you could save that much by doing the upload to S3 yourself.
My other concern with this is that if the tarsnap server ever goes away, customers risk losing their data, since the server maintains the mapping of S3 objects to blobs. That's worrying. Assurances are not mechanisms.