I understand why this policy is in place: they have "unlimited" storage at a fixed price, so their ROI increases the fewer files they store. But the policy of deleting files no longer on my hard drive after 30 days renders Mozy, Backblaze and Carbonite unusable for offsite backup. I'm sure they're great for temporary storage, but you might get a very unpleasant surprise trying to recover files which have been deleted due to this policy.
I'll stick to offsite hard drives kept at the office. I'll sleep better.
Just to be clear, if the data is still on your local drive, then it's kept on the online backup, no matter how old the file. It's only deleted files that get tossed after 30 days.
I think this situation depends entirely on how you look at it. If one thinks of Mozy/Backblaze/Carbonite as an online Time Machine drive (to use the Mac example with which I am most familiar), then their policy of deleting files
that you have deleted is the same as your local Time Machine backup, which eventually deletes files that are no longer on the data drive.
After all, I generate many gigabytes of data every week -- temp files, working files, huge layered Photoshop files in post production -- that I will delete from my hard drive when I am done working on them. If Mozy had to archive all of those files indefinitely for everyone until the end of time, well, there is not enough drive space in the cloud for all that. I certainly don't have room for it all, so I have to be moderately selective in what I save. And be very careful about what I delete.
I agree that keeping your own hard drives offsite can be better in several ways -- they are easier to access, and you can store whatever you like. However, if you follow a common backup plan, and clone your data drives on a weekly rotating basis, so you have 3 or 4 drives in rotation, eventually your deleted files will drop out of the rotation and be lost forever. In, say, a month or so.
The only way to prevent that is to: (1) never delete anything, and (2) keep adding hard drives and their clones to your on-site and off-site storage.
The final reality is that there is no perfect backup plan. Everything is a compromise, and no matter what system is used, it's possible to lose data through carelessness, viruses, and acts of Nature.