Development time. Why invest time in such optimizations instead of other features when the app is fast enough? Time is not infinite and we need to prioritize what takes our development time.
I'd say time spent optimizing grep (GNU or otherwise) is always well spent. Saving a clock cycle or two per execution in my work's enterprise would translate into needing to lease less hardware, based just on how much grep is used. I imagine the same is true of all large Unix-y enterprises.
