If a ciphertext is not authenticated, it can be trivially tampered with. This means that someone with access to the encrypted drive could add backdoors or otherwise tamper with the executables and data even though it is encrypted.
Shouldn't it be impossible to forge a plaintext without the key for a good encryption algorithm?
I imagine a good algorithm not to be just key -> pseudorandom stuff that is XORed with data, but something that has cascading. Change a bit anywhere, and a whole block changes unpredictably. Include the physical position of the block in the key, so that it impossible to copy blocks around to duplicate data.
With 4k native drives, this became completely impractical. To keep ratios similar, you would have to present 32k byte devices up the stack, which filesystems have troubles with. Or have 1 MAC sector per data sector or similar, cutting your storage in half.
Here's a short thread about the problems with adding it to dm-crypt: http://comments.gmane.org/gmane.linux.kernel.device-mapper.d...
I'm sure you can find more threads if you look around.