Active shooter scenario pops to mind for it's usefulness. If I'm huddled down somewhere, trying not to be noticed, I'd much rather text 911 telling them what's going on, how many shooters there are, where they are, what they're armed with, etc. rather than calling and just keeping the line open and hoping you figure out where I am.
Same with a home invasion. I want to keep quiet so they don't know I'm there/where I am. Texting would be useful.
If you look at actual active shooter incidents, such as Columbine or Virginia Tech, it seems the 911 system as it is worked just fine. For example, at Columbine the shooting started at 11:19-11:20. First 911 call was 11:21, and first officer was on scene at 11:24. It didn't take him long to determine who the suspects were or where they were because shots were exchanged just after that.
True home invasions... exeedingly rare. Most of the time burglars are surprised when they find someone home and run away as soon as they realize it.
I'm not saying there are NO circumstances in which texting 911 would be helpful, I just think those situations are very rare and the benefits must be weighed against the negatives and cost.
You may all think of the 1 in a 1000 active shooter or home invasion call, I think of the fact that someone calling to report a run-of-the-mill accident will now text me instead of calling me, distracting them from driving safely. I think about people who will text something like what Sasha said, and how it will now take me 8x as long to decipher what they've said and will lose the ability to just ask them the questions I need to know. Most people are much more focused on "he's just so rude to me and he used to be so nice, and since my aunt died I don't know what to do anymore, and he doesn't take good care of our daughter, and he cheated on me with blah blah blah" and that is what they are going to text. More time wasted that could be spent more efficiently just taking calls and getting my answers in real time, and moving on to that next 911 line that's already ringing.
If you spend enough time in a very busy 911 center, you would understand. I routinely have to put several 911 callers on hold to answer another one of the other 10+ 911 lines ringing. I don't think having to deal with "fund racon in hse wat do i do?" and such is going to help me or anyone else who is calling for my help.