I do agree with you to some extent Dduck.
Incremental backups are a real boon in many situations, but I find that most users end up using them incorrectly, which is a shame.
"As to the point"
Long term storage on Flash drives is even more susceptible to bitrot than it is on a platter drive. (and it is physically more susceptible to breakage, failure and loss) I would suggest a full refresh once a month if going that route (ie delete and recopy)
One reason I was interested in the mesh sync, is it would negate the large library if backup time is incremental, and also alleviate any disk problems as it would be cloud based (you gota bet they have it on multiple servers)
Still any back up is better than none.
Two locations are better still
Skydrive is a bit of a bust.
Using the web based service, you can only upload mulitple files if there in the same folder. (1000 at a time) plus a 50mb per file size limit
I mapped the drive to windows, but then book files throw up errors - think they only want you to use it for office files.
There is a way to do it though, by exporting all your books to a single directory and then uploding it like that, not very pretty, and you loose opf and cover, but at least your books are safe (worth it if you don't have a backup elsewhere)
Paying for dropbox or similar may be better option, depending on your library size