This page has their three plans. One is free, but it assumes your backup locations are not Crashplan's systems (friends, family, other computers you control). That's probably the catch you're looking for; you have to come up with the backup location yourself. Being able to back up one system to CP's servers (unlimited quantity) is on the order of 5 a month; being able to back up 2-10 systems is on the order of 15.
You may also consider the encryption options to be a catch; as you note, the free plan has less encryption. On the other hand, the data is being stored somewhere you presumably trust. The plans that save to CP's systems have a couple of key management options, and in one they don't get the key (barring sneakiness, but if you're that worried about it you're likely going to rig it so what they're backing up is encrypted already).
Personally, I back up one system (my file server) to Crashplan, and most of my other ones to the file server. NOTE! This does not mean my other systems are getting backed up to Crashplan indirectly! If I want it backed up to Crashplan I have to copy it to the fileserver as a regular file. It just means that stuff on my laptop that I don't think is important enough to go offsite still gets automatically backed up to a different disk.
I've thought about going tape for local backup, but I never felt comfortable with it, because I have the impression that the lifespan of tape is poor. What's your experience in tape lifespan (which I suppose also involves how often/whether you rewrite instead of getting a new tape)?
(Currenly local backup for most of my stuff is "put it on the file server", where it's mirrored, which is of course not perfect. But the file server is also backed up offsite through CrashPlan.)
I think that perhaps it's not as firmly equivalent as you imply; a concept in a book cannot be used in the same ways as a concept in human memory without being copied to human memory. At which point it's not the concept in the book getting used any more.