Is Truecrypt use of cascading encryption safe? Is it useful? Truecrypt is arguably one of the most popular and widely used encryption applications in use today, yet it seems to use a very unconventional scheme.
The software offers cascading encryption, where the user can specify either double or triple encryption using any combination of AES, Twofish, and Serpent. Unusually, multiple-encryption is performed on each block, before moving on to the next block, as opposed to encrypting all the data and then encrypting the resulting ciphertext.
After having spent a lot of time reading through crypto.se, the consensus seems to be that multiple/cascading encryption offers little (or perhaps no) extra security, and some even argue that if may be weaker than just following a standard protocol (although the latter rarely seems to be substantiated by any reasoning). However, in Bruce Schneier's Applied Cryptography (considered by many to be a quintessential text on cryptography), he advocates the use of multiple encryption (albeit cautiously, and with some caveats). See sections: 15.7 Cascading Multiple Block Algorithms, 15.8 Combining Multiple Block Algorithms, and 17.11 Cascading Multiple Stream Ciphers.
Another example of going against this consensus would be the Tahoe-Lafs 100 Year Cryptography Project, which has been recommended on crypto.se. Their protocol involves encrypting with AES in CTR mode and then encrypting the output with xSalsa20.
So my question is, is Truecrypt's use of multiple/cascading cryptography reasonable, and why?