The main reason to use grep is portability/availability. While a modern improvement to grep (yes, I'm an ack user and fan too) you can't simply depend on it in portable scripts and such.

grep is defined for POSIX after all: http://pubs.opengroup.org/onlinepubs/009695399/utilities/gre...

I also suspect (without looking) that there are obscure (but occasionally useful) grep features that ack doesn't support.