>two identical encrypted files.

Good practice for file encryption is to pad some random bytes to the start of a file. This makes known cyphertext attacks much harder.

So encrypting identical files should result in different files and also slightly different length files.

How about if a single byte changes? Now the encrypted output looks 100% unique, and you have to treat it as an entirely new file. You lose the ability to do diffs on a file by file basis or proper deduplication across all files.

That's defeating the purpose of encrypting them.

If you don't CBC, wherever is on the cloud side will have two files with one block off. Let's assume the file is a txt file. For smaller cipher block sizes, it is becoming very easy to guess your encryption key.

Tldr, you want any attacker to lose any diff ability on your encrypted data.

That depends on how you're storing the files. I was really just trying to highlight that for deduplication across files you need to deduplicate before you encrypt.

Guess the encryption key? That shouldn't be possible, even if you use something as terrible as ECB.