Advantages of making streamcipher stream depend on plaintext?
Most advantages have to do with the fact that it includes authentication. For example:


An authenticated encryption primitive is easier to use correctly.
Only a single primitive that has to be secure.
One pass over the data to both encrypt and authenticate may be faster. (Bernstein's rebuttal is that a separate MAC allows faster detection of forgeries, mitigating denial of service attacks.)


There is one potential benefit I can think of that isn't related to authentication, but it should never come into play if you are using the cipher securely:


A plaintext dependent keystream may recover from IV collisions.


This is similar to how a block cipher in CFB mode can be secure after the first block, even with duplicate IVs, if the plaintext differs.