It seems silly, but texting is significantly more discreet than a call, and takes less effort to do.
If you're within earshot of a "bad guy," you can nonchalantly send a text, and it can be done a lot quicker.
Isn't reducing bystander apathy a good thing?
It would also help for situations where there is a lot of noise, or you have been injured such that you can't talk.
I would imagine that handling 100 people texting 911 would be significantly easier then handling 100 people calling it. That's probably good news for any large scale emergencies.