Not giving away the length is mainly an assistance to people with really short passwords. Knowing that someone has a 12 character password doesn't help attackers much, but knowing that someone has a 6 character password would be really useful.
It's still not very useful to hide the length. If you don't know the length and just start guessing with passwords of length 0 it only adds about 1/N extra guesses where N is the alphabet size compared to guessing strictly the right length. So it is a very small savings to know the password length.
It might matter a bit more for dictionary-based attacks (you don't have to bother hashing dictionary permutations that don't match the expected length) but I still suspect it doesn't save you much.
For opportunistic attacks, this could help you identify those with short passwords and only attack them. This is a factor of N speedup where N is the pool of people you are interested in attacking.
