I agree it is not necessary for sendfile() (you were right).
Do not introducing it for send(), though, poses some questions. 
For instance, should we deprecate or ignore 'blocksize' argument in ftplib as well?
Generally speaking, when using send() there are circumstances where you might want to adjust the number of bytes to read from the file, for instance:

- 1: set a small blocksize (say 512 bytes) on systems where you have a limited amount of memory

- 2: set a big blocksize (say 256000) in order to speed up the transfer / use less CPU cycles; on very fast networks (e.g. LANs) this may result in a considerable speedup (I know 'cause I conducted these kind of tests in pyftpdlib: https://code.google.com/p/pyftpdlib/issues/detail?id=94).