I don't know about SpiderOak but I totally trust Tarsnap.
But agreed we have to trust the software companies/devs when using crypto.
Not many people really trust Dropbox anymore to build client-side encryption themselves. But you could use a different program for encryption with dropbox as simple storage. That being said, you still have to trust the Dropbox binaries installed on your system as well. Security paranoia can go deep.
IIRC, Dropbox has HTTP-based API that allows basic file operations, like uploading, downloading and listing files. So, proprietary client isn't really required.
Well, if someone would code a Dropbox-based TAHOE-LAFS backend, I'd seriously consider really using Dropbox. (My phone came with 1-year "free" 50GB offer, so I'm using it for some completely non-private files, like my cat's photos I've publicly shared.)
But agreed we have to trust the software companies/devs when using crypto.
Not many people really trust Dropbox anymore to build client-side encryption themselves. But you could use a different program for encryption with dropbox as simple storage. That being said, you still have to trust the Dropbox binaries installed on your system as well. Security paranoia can go deep.