Their profitability depends on being able to dedupe and compress data across all customers. They would need to raise prices significantly to make client side encryption a built in feature.

You trust the end users to back up their private keys?

I don't care what other people do. I want client side encryption!

Then use tarsnap already. Bonus: it is owned by our own cperciva.

tarsnap is nerdish, secure and reasonably priced for what it provides IIRC. Haven't used it though but expect someone would have yelled out here if it was bad. (In fact the only one I've seen bashing it was patio11, -because it was too cheap and too nerdish.)

tptacek also complains that tarsnap is too nerdish. :-)

At least on Linux/OSX/Android, you can use encfs to have a encrypted directory tree.

Spideroak can do it without private keys to back up.